Remove MediaSourceEventListener base class from AdsMediaSource.EventListener

The ad events are independent of the other media source events. Also,
registering the listener to the internal ad media sources will report the
regular media source events twice: once directly (with a non-ad media period
id) and once through the wrapping ads source (with the correct ad media period
id).

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=189905561

library/core/src/main/java/com/google/android/exoplayer2/source/ads/AdsMediaSource.java

@@ -30,7 +30,6 @@ import com.google.android.exoplayer2.source.ExtractorMediaSource;
 import com.google.android.exoplayer2.source.MediaPeriod;
 import com.google.android.exoplayer2.source.MediaSource;
 import com.google.android.exoplayer2.source.MediaSource.MediaPeriodId;
-import com.google.android.exoplayer2.source.MediaSourceEventListener;
 import com.google.android.exoplayer2.upstream.Allocator;
 import com.google.android.exoplayer2.upstream.DataSource;
 import com.google.android.exoplayer2.util.Assertions;
@@ -66,7 +65,7 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
   }
 
   /** Listener for ads media source events. */
-  public interface EventListener extends MediaSourceEventListener {
+  public interface EventListener {
 
     /**
      * Called if there was an error loading one or more ads. The loader will skip the problematic
@@ -221,7 +220,6 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
       if (adGroupMediaSources[adGroupIndex].length <= adIndexInAdGroup) {
         Uri adUri = adPlaybackState.adGroups[id.adGroupIndex].uris[id.adIndexInAdGroup];
         MediaSource adMediaSource = adMediaSourceFactory.createMediaSource(adUri);
-        adMediaSource.addEventListener(eventHandler, eventListener);
         int oldAdCount = adGroupMediaSources[id.adGroupIndex].length;
         if (adIndexInAdGroup >= oldAdCount) {
           int adCount = adIndexInAdGroup + 1;