Add some missing @Nullable to public API to improve Kotlin compatiblity.

Kotlin will throw NPE whenever a method returns null or an interface is
called with a null parameter and the respective values are not marked as
@Nullable.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=213262886

library/core/src/main/java/com/google/android/exoplayer2/SimpleExoPlayer.java

@@ -296,7 +296,7 @@ public class SimpleExoPlayer
   }
 
   @Override
-  public void setVideoSurface(Surface surface) {
+  public void setVideoSurface(@Nullable Surface surface) {
     removeSurfaceCallbacks();
     setVideoSurfaceInternal(surface, false);
     int newSurfaceSize = surface == null ? 0 : C.LENGTH_UNSET;
@@ -828,7 +828,7 @@ public class SimpleExoPlayer
   }
 
   @Override
-  public ExoPlaybackException getPlaybackError() {
+  public @Nullable ExoPlaybackException getPlaybackError() {
     return player.getPlaybackError();
   }
 
@@ -1028,7 +1028,7 @@ public class SimpleExoPlayer
   }
 
   @Override
-  public Object getCurrentManifest() {
+  public @Nullable Object getCurrentManifest() {
     return player.getCurrentManifest();
   }
 
@@ -1134,7 +1134,7 @@ public class SimpleExoPlayer
     }
   }
 
-  private void setVideoSurfaceInternal(Surface surface, boolean ownsSurface) {
+  private void setVideoSurfaceInternal(@Nullable Surface surface, boolean ownsSurface) {
     // Note: We don't turn this method into a no-op if the surface is being replaced with itself
     // so as to ensure onRenderedFirstFrame callbacks are still called in this case.
     List<PlayerMessage> messages = new ArrayList<>();

library/core/src/main/java/com/google/android/exoplayer2/analytics/AnalyticsCollector.java

@@ -299,7 +299,7 @@ public class AnalyticsCollector
   }
 
   @Override
-  public final void onRenderedFirstFrame(Surface surface) {
+  public final void onRenderedFirstFrame(@Nullable Surface surface) {
     EventTime eventTime = generateReadingMediaPeriodEventTime();
     for (AnalyticsListener listener : listeners) {
       listener.onRenderedFirstFrame(eventTime, surface);

library/core/src/main/java/com/google/android/exoplayer2/analytics/AnalyticsListener.java

@@ -448,7 +448,7 @@ public interface AnalyticsListener {
    * @param surface The {@link Surface} to which a first frame has been rendered, or {@code null} if
    *     the renderer renders to something that isn't a {@link Surface}.
    */
-  default void onRenderedFirstFrame(EventTime eventTime, Surface surface) {}
+  default void onRenderedFirstFrame(EventTime eventTime, @Nullable Surface surface) {}
 
   /**
    * Called each time a drm session is acquired.

library/core/src/main/java/com/google/android/exoplayer2/util/EventLogger.java

@@ -323,8 +323,8 @@ public class EventLogger implements AnalyticsListener {
   }
 
   @Override
-  public void onRenderedFirstFrame(EventTime eventTime, Surface surface) {
-    logd(eventTime, "renderedFirstFrame", surface.toString());
+  public void onRenderedFirstFrame(EventTime eventTime, @Nullable Surface surface) {
+    logd(eventTime, "renderedFirstFrame", String.valueOf(surface));
   }
 
   @Override

library/core/src/main/java/com/google/android/exoplayer2/video/VideoRendererEventListener.java

@@ -96,7 +96,7 @@ public interface VideoRendererEventListener {
    * @param surface The {@link Surface} to which a first frame has been rendered, or {@code null} if
    *     the renderer renders to something that isn't a {@link Surface}.
    */
-  void onRenderedFirstFrame(Surface surface);
+  void onRenderedFirstFrame(@Nullable Surface surface);
 
   /**
    * Called when the renderer is disabled.
@@ -124,20 +124,16 @@ public interface VideoRendererEventListener {
       this.listener = listener;
     }
 
-    /**
-     * Invokes {@link VideoRendererEventListener#onVideoEnabled(DecoderCounters)}.
-     */
-    public void enabled(final DecoderCounters decoderCounters) {
+    /** Invokes {@link VideoRendererEventListener#onVideoEnabled(DecoderCounters)}. */
+    public void enabled(DecoderCounters decoderCounters) {
       if (listener != null) {
         handler.post(() -> listener.onVideoEnabled(decoderCounters));
       }
     }
 
-    /**
-     * Invokes {@link VideoRendererEventListener#onVideoDecoderInitialized(String, long, long)}.
-     */
-    public void decoderInitialized(final String decoderName,
-        final long initializedTimestampMs, final long initializationDurationMs) {
+    /** Invokes {@link VideoRendererEventListener#onVideoDecoderInitialized(String, long, long)}. */
+    public void decoderInitialized(
+        String decoderName, long initializedTimestampMs, long initializationDurationMs) {
       if (listener != null) {
         handler.post(
             () ->
@@ -146,29 +142,26 @@ public interface VideoRendererEventListener {
       }
     }
 
-    /**
-     * Invokes {@link VideoRendererEventListener#onVideoInputFormatChanged(Format)}.
-     */
-    public void inputFormatChanged(final Format format) {
+    /** Invokes {@link VideoRendererEventListener#onVideoInputFormatChanged(Format)}. */
+    public void inputFormatChanged(Format format) {
       if (listener != null) {
         handler.post(() -> listener.onVideoInputFormatChanged(format));
       }
     }
 
-    /**
-     * Invokes {@link VideoRendererEventListener#onDroppedFrames(int, long)}.
-     */
-    public void droppedFrames(final int droppedFrameCount, final long elapsedMs) {
+    /** Invokes {@link VideoRendererEventListener#onDroppedFrames(int, long)}. */
+    public void droppedFrames(int droppedFrameCount, long elapsedMs) {
       if (listener != null) {
         handler.post(() -> listener.onDroppedFrames(droppedFrameCount, elapsedMs));
       }
     }
 
-    /**
-     * Invokes {@link VideoRendererEventListener#onVideoSizeChanged(int, int, int, float)}.
-     */
-    public void videoSizeChanged(final int width, final int height,
-        final int unappliedRotationDegrees, final float pixelWidthHeightRatio) {
+    /** Invokes {@link VideoRendererEventListener#onVideoSizeChanged(int, int, int, float)}. */
+    public void videoSizeChanged(
+        int width,
+        int height,
+        final int unappliedRotationDegrees,
+        final float pixelWidthHeightRatio) {
       if (listener != null) {
         handler.post(
             () ->
@@ -177,19 +170,15 @@ public interface VideoRendererEventListener {
       }
     }
 
-    /**
-     * Invokes {@link VideoRendererEventListener#onRenderedFirstFrame(Surface)}.
-     */
-    public void renderedFirstFrame(final Surface surface) {
+    /** Invokes {@link VideoRendererEventListener#onRenderedFirstFrame(Surface)}. */
+    public void renderedFirstFrame(@Nullable Surface surface) {
       if (listener != null) {
         handler.post(() -> listener.onRenderedFirstFrame(surface));
       }
     }
 
-    /**
-     * Invokes {@link VideoRendererEventListener#onVideoDisabled(DecoderCounters)}.
-     */
-    public void disabled(final DecoderCounters counters) {
+    /** Invokes {@link VideoRendererEventListener#onVideoDisabled(DecoderCounters)}. */
+    public void disabled(DecoderCounters counters) {
       if (listener != null) {
         handler.post(
             () -> {