Allow reusing SingleSampleMediaSource.

GitHub:#3498

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=183391117

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

@@ -158,8 +158,6 @@ public final class SingleSampleMediaSource implements MediaSource {
   private final boolean treatLoadErrorsAsEndOfStream;
   private final Timeline timeline;
 
-  private boolean isPrepared;
-
   /**
    * @param uri The {@link Uri} of the media stream.
    * @param dataSourceFactory The factory from which the {@link DataSource} to read the media will
@@ -253,8 +251,6 @@ public final class SingleSampleMediaSource implements MediaSource {
 
   @Override
   public void prepareSource(ExoPlayer player, boolean isTopLevelSource, Listener listener) {
-    Assertions.checkState(!isPrepared, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
-    isPrepared = true;
     listener.onSourceInfoRefreshed(this, timeline, null);
   }