Allow setting player lazily in AnalyticsCollector.

This helps to use the AnalyticsCollector without SimpleExoPlayer. Currently,
that may be problematic, if the contructor needs the player, but in order to
create the player, one already needs the AnalyticsCollector as a listener for
the renderers.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=199105012

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

@@ -46,6 +46,7 @@ import java.util.Collections;
 import java.util.List;
 import java.util.Set;
 import java.util.concurrent.CopyOnWriteArraySet;
+import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
 
 /**
  * Data collector which is able to forward analytics events to {@link AnalyticsListener}s by
@@ -66,29 +67,34 @@ public class AnalyticsCollector
     /**
      * Creates an analytics collector for the specified player.
      *
-     * @param player The {@link Player} for which data will be collected.
+     * @param player The {@link Player} for which data will be collected. Can be null, if the player
+     *     is set by calling {@link AnalyticsCollector#setPlayer(Player)} before using the analytics
+     *     collector.
      * @param clock A {@link Clock} used to generate timestamps.
      * @return An analytics collector.
      */
-    public AnalyticsCollector createAnalyticsCollector(Player player, Clock clock) {
+    public AnalyticsCollector createAnalyticsCollector(@Nullable Player player, Clock clock) {
       return new AnalyticsCollector(player, clock);
     }
   }
 
   private final CopyOnWriteArraySet<AnalyticsListener> listeners;
-  private final Player player;
   private final Clock clock;
   private final Window window;
   private final MediaPeriodQueueTracker mediaPeriodQueueTracker;
 
+  private @MonotonicNonNull Player player;
+
   /**
    * Creates an analytics collector for the specified player.
    *
-   * @param player The {@link Player} for which data will be collected.
+   * @param player The {@link Player} for which data will be collected. Can be null, if the player
+   *     is set by calling {@link AnalyticsCollector#setPlayer(Player)} before using the analytics
+   *     collector.
    * @param clock A {@link Clock} used to generate timestamps.
    */
-  protected AnalyticsCollector(Player player, Clock clock) {
-    this.player = Assertions.checkNotNull(player);
+  protected AnalyticsCollector(@Nullable Player player, Clock clock) {
+    this.player = player;
     this.clock = Assertions.checkNotNull(clock);
     listeners = new CopyOnWriteArraySet<>();
     mediaPeriodQueueTracker = new MediaPeriodQueueTracker();
@@ -113,6 +119,17 @@ public class AnalyticsCollector
     listeners.remove(listener);
   }
 
+  /**
+   * Sets the player for which data will be collected. Must only be called if no player has been set
+   * yet.
+   *
+   * @param player The {@link Player} for which data will be collected.
+   */
+  public void setPlayer(Player player) {
+    Assertions.checkState(this.player == null);
+    this.player = Assertions.checkNotNull(player);
+  }
+
   // External events.
 
   /**
@@ -541,6 +558,7 @@ public class AnalyticsCollector
 
   /** Returns a new {@link EventTime} for the specified window index and media period id. */
   protected EventTime generateEventTime(int windowIndex, @Nullable MediaPeriodId mediaPeriodId) {
+    Assertions.checkNotNull(player);
     long realtimeMs = clock.elapsedRealtime();
     Timeline timeline = player.getCurrentTimeline();
     long eventPositionMs;
@@ -579,7 +597,7 @@ public class AnalyticsCollector
 
   private EventTime generateEventTime(@Nullable WindowAndMediaPeriodId mediaPeriod) {
     if (mediaPeriod == null) {
-      int windowIndex = player.getCurrentWindowIndex();
+      int windowIndex = Assertions.checkNotNull(player).getCurrentWindowIndex();
       MediaPeriodId mediaPeriodId = mediaPeriodQueueTracker.tryResolveWindowIndex(windowIndex);
       return generateEventTime(windowIndex, mediaPeriodId);
     }