Add @Nullable to ExoMediaDrm's setListener methods

These methods document themselves only by referencing the Android's
MediaDrm, which explicitly accepts null [1]. The implementation in
FrameworkMediaDrm also handles nulls.

[1] https://developer.android.com/reference/android/media/MediaDrm#setOnEventListener(android.media.MediaDrm.OnEventListener)

PiperOrigin-RevId: 302007441

library/core/src/main/java/com/google/android/exoplayer2/drm/DummyExoMediaDrm.java

@@ -34,17 +34,17 @@ public final class DummyExoMediaDrm implements ExoMediaDrm {
   }
   }
 
 
   @Override
   @Override
-  public void setOnEventListener(OnEventListener listener) {
+  public void setOnEventListener(@Nullable OnEventListener listener) {
     // Do nothing.
     // Do nothing.
   }
   }
 
 
   @Override
   @Override
-  public void setOnKeyStatusChangeListener(OnKeyStatusChangeListener listener) {
+  public void setOnKeyStatusChangeListener(@Nullable OnKeyStatusChangeListener listener) {
     // Do nothing.
     // Do nothing.
   }
   }
 
 
   @Override
   @Override
-  public void setOnExpirationUpdateListener(OnExpirationUpdateListener listener) {
+  public void setOnExpirationUpdateListener(@Nullable OnExpirationUpdateListener listener) {
     // Do nothing.
     // Do nothing.
   }
   }
 
 

library/core/src/main/java/com/google/android/exoplayer2/drm/ExoMediaDrm.java

@@ -225,13 +225,13 @@ public interface ExoMediaDrm {
   }
   }
 
 
   /** @see MediaDrm#setOnEventListener(MediaDrm.OnEventListener) */
   /** @see MediaDrm#setOnEventListener(MediaDrm.OnEventListener) */
-  void setOnEventListener(OnEventListener listener);
+  void setOnEventListener(@Nullable OnEventListener listener);
 
 
   /** @see MediaDrm#setOnKeyStatusChangeListener(MediaDrm.OnKeyStatusChangeListener, Handler) */
   /** @see MediaDrm#setOnKeyStatusChangeListener(MediaDrm.OnKeyStatusChangeListener, Handler) */
-  void setOnKeyStatusChangeListener(OnKeyStatusChangeListener listener);
+  void setOnKeyStatusChangeListener(@Nullable OnKeyStatusChangeListener listener);
 
 
   /** @see MediaDrm#setOnExpirationUpdateListener(MediaDrm.OnExpirationUpdateListener, Handler) */
   /** @see MediaDrm#setOnExpirationUpdateListener(MediaDrm.OnExpirationUpdateListener, Handler) */
-  void setOnExpirationUpdateListener(OnExpirationUpdateListener listener);
+  void setOnExpirationUpdateListener(@Nullable OnExpirationUpdateListener listener);
 
 
   /** @see MediaDrm#openSession() */
   /** @see MediaDrm#openSession() */
   byte[] openSession() throws MediaDrmException;
   byte[] openSession() throws MediaDrmException;

library/core/src/main/java/com/google/android/exoplayer2/drm/FrameworkMediaDrm.java

@@ -106,7 +106,7 @@ public final class FrameworkMediaDrm implements ExoMediaDrm {
   }
   }
 
 
   @Override
   @Override
-  public void setOnEventListener(ExoMediaDrm.OnEventListener listener) {
+  public void setOnEventListener(@Nullable ExoMediaDrm.OnEventListener listener) {
     mediaDrm.setOnEventListener(
     mediaDrm.setOnEventListener(
         listener == null
         listener == null
             ? null
             ? null
@@ -115,7 +115,8 @@ public final class FrameworkMediaDrm implements ExoMediaDrm {
   }
   }
 
 
   @Override
   @Override
-  public void setOnKeyStatusChangeListener(ExoMediaDrm.OnKeyStatusChangeListener listener) {
+  public void setOnKeyStatusChangeListener(
+      @Nullable ExoMediaDrm.OnKeyStatusChangeListener listener) {
     if (Util.SDK_INT < 23) {
     if (Util.SDK_INT < 23) {
       throw new UnsupportedOperationException();
       throw new UnsupportedOperationException();
     }
     }
@@ -135,7 +136,7 @@ public final class FrameworkMediaDrm implements ExoMediaDrm {
   }
   }
 
 
   @Override
   @Override
-  public void setOnExpirationUpdateListener(OnExpirationUpdateListener listener) {
+  public void setOnExpirationUpdateListener(@Nullable OnExpirationUpdateListener listener) {
     if (Util.SDK_INT < 23) {
     if (Util.SDK_INT < 23) {
       throw new UnsupportedOperationException();
       throw new UnsupportedOperationException();
     }
     }