Clarify doc for Player.getDuration and add Player.getContentDuration.

Similar to getBufferedPosition and getCurrentPosition, getDuration should
mention that it also returns the duration of the current ad.

And, also similar to getContentBufferedDuration and getContentPosition, a
new method getContentDuration simplifies querying the content duration while
playing an ad.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=209914696

extensions/cast/src/main/java/com/google/android/exoplayer2/ext/cast/CastPlayer.java

@@ -568,6 +568,11 @@ public final class CastPlayer implements Player {
   }
 
   @Override
+  public long getContentDuration() {
+    return getDuration();
+  }
+
+  @Override
   public boolean isLoading() {
     return false;
   }

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

@@ -494,18 +494,13 @@ import java.util.concurrent.CopyOnWriteArraySet;
 
   @Override
   public long getDuration() {
-    Timeline timeline = playbackInfo.timeline;
-    if (timeline.isEmpty()) {
-      return C.TIME_UNSET;
-    }
     if (isPlayingAd()) {
       MediaPeriodId periodId = playbackInfo.periodId;
-      timeline.getPeriodByUid(periodId.periodUid, period);
+      playbackInfo.timeline.getPeriodByUid(periodId.periodUid, period);
       long adDurationUs = period.getAdDurationUs(periodId.adGroupIndex, periodId.adIndexInAdGroup);
       return C.usToMs(adDurationUs);
-    } else {
-      return timeline.getWindow(getCurrentWindowIndex(), window).getDurationMs();
     }
+    return getContentDuration();
   }
 
   @Override
@@ -571,6 +566,13 @@ import java.util.concurrent.CopyOnWriteArraySet;
   }
 
   @Override
+  public long getContentDuration() {
+    return playbackInfo.timeline.isEmpty()
+        ? C.TIME_UNSET
+        : playbackInfo.timeline.getWindow(getCurrentWindowIndex(), window).getDurationMs();
+  }
+
+  @Override
   public long getContentPosition() {
     if (isPlayingAd()) {
       playbackInfo.timeline.getPeriodByUid(playbackInfo.periodId.periodUid, period);

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

@@ -748,8 +748,8 @@ public interface Player {
   @Nullable Object getCurrentTag();
 
   /**
-   * Returns the duration of the current window in milliseconds, or {@link C#TIME_UNSET} if the
-   * duration is not known.
+   * Returns the duration of the current content window or ad in milliseconds, or {@link
+   * C#TIME_UNSET} if the duration is not known.
    */
   long getDuration();
 
@@ -808,6 +808,13 @@ public interface Player {
   int getCurrentAdIndexInAdGroup();
 
   /**
+   * If {@link #isPlayingAd()} returns {@code true}, returns the duration of the current content
+   * window in milliseconds, or {@link C#TIME_UNSET} if the duration is not known. If there is no ad
+   * playing, the returned duration is the same as that returned by {@link #getDuration()}.
+   */
+  long getContentDuration();
+
+  /**
    * If {@link #isPlayingAd()} returns {@code true}, returns the content position that will be
    * played once all ads in the ad group have finished playing, in milliseconds. If there is no ad
    * playing, the returned position is the same as that returned by {@link #getCurrentPosition()}.

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

@@ -1054,6 +1054,11 @@ public class SimpleExoPlayer
   }
 
   @Override
+  public long getContentDuration() {
+    return player.getContentDuration();
+  }
+
+  @Override
   public long getContentPosition() {
     return player.getContentPosition();
   }

testutils_robolectric/src/main/java/com/google/android/exoplayer2/testutil/StubExoPlayer.java

@@ -300,6 +300,11 @@ public abstract class StubExoPlayer implements ExoPlayer {
   }
 
   @Override
+  public long getContentDuration() {
+    throw new UnsupportedOperationException();
+  }
+
+  @Override
   public long getContentPosition() {
     throw new UnsupportedOperationException();
   }