Fix ImaAdsMediaSource isTopLevelSource

AdsMediaSource must be top-level. Currently the (deprecated) ImaAdsMediaSource
can't be used because it prepares its contained AdsMediaSource as a child
source.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=186990882

extensions/ima/src/main/java/com/google/android/exoplayer2/ext/ima/ImaAdsMediaSource.java

@@ -20,23 +20,22 @@
 import android.view.ViewGroup;
 import com.google.android.exoplayer2.ExoPlayer;
 import com.google.android.exoplayer2.Timeline;
-import com.google.android.exoplayer2.source.CompositeMediaSource;
 import com.google.android.exoplayer2.source.MediaPeriod;
 import com.google.android.exoplayer2.source.MediaSource;
 import com.google.android.exoplayer2.source.ads.AdsMediaSource;
 import com.google.android.exoplayer2.upstream.Allocator;
 import com.google.android.exoplayer2.upstream.DataSource;
+import java.io.IOException;
 
 /**
  * A {@link MediaSource} that inserts ads linearly with a provided content media source.
  *
  * @deprecated Use com.google.android.exoplayer2.source.ads.AdsMediaSource with ImaAdsLoader.
  */
 @Deprecated
-public final class ImaAdsMediaSource extends CompositeMediaSource<Void> {
+public final class ImaAdsMediaSource implements MediaSource {
 
   private final AdsMediaSource adsMediaSource;
-  private Listener listener;
 
   /**
    * Constructs a new source that inserts ads linearly with the content specified by
@@ -75,10 +74,23 @@ public ImaAdsMediaSource(
   }
 
   @Override
-  public void prepareSource(ExoPlayer player, boolean isTopLevelSource, Listener listener) {
-    super.prepareSource(player, isTopLevelSource, listener);
-    this.listener = listener;
-    prepareChildSource(/* id= */ null, adsMediaSource);
+  public void prepareSource(
+      final ExoPlayer player, boolean isTopLevelSource, final Listener listener) {
+    adsMediaSource.prepareSource(
+        player,
+        isTopLevelSource,
+        new Listener() {
+          @Override
+          public void onSourceInfoRefreshed(
+              MediaSource source, Timeline timeline, @Nullable Object manifest) {
+            listener.onSourceInfoRefreshed(ImaAdsMediaSource.this, timeline, manifest);
+          }
+        });
+  }
+
+  @Override
+  public void maybeThrowSourceInfoRefreshError() throws IOException {
+    adsMediaSource.maybeThrowSourceInfoRefreshError();
   }
 
   @Override
@@ -92,8 +104,7 @@ public void releasePeriod(MediaPeriod mediaPeriod) {
   }
 
   @Override
-  protected void onChildSourceInfoRefreshed(
-      Void id, MediaSource mediaSource, Timeline timeline, @Nullable Object manifest) {
-    listener.onSourceInfoRefreshed(this, timeline, manifest);
+  public void releaseSource() {
+    adsMediaSource.releaseSource();
   }
 }