Allow reusing HlsMediaSource.

GitHub:#3498

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=183373647

library/hls/src/main/java/com/google/android/exoplayer2/source/hls/HlsMediaSource.java

@@ -324,10 +324,9 @@ public final class HlsMediaSource implements MediaSource,
 
   @Override
   public void prepareSource(ExoPlayer player, boolean isTopLevelSource, Listener listener) {
-    Assertions.checkState(sourceListener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
+    sourceListener = listener;
     playlistTracker = new HlsPlaylistTracker(manifestUri, dataSourceFactory, eventDispatcher,
         minLoadableRetryCount, this, playlistParser);
-    sourceListener = listener;
     playlistTracker.start();
   }
 
@@ -361,6 +360,7 @@ public final class HlsMediaSource implements MediaSource,
       playlistTracker.release();
       playlistTracker = null;
     }
+    sourceListener = null;
   }
 
   @Override