Make FakeMediaSource reusable.

GitHub:#3498

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=183392095

testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeMediaSource.java

@@ -78,6 +78,7 @@ public class FakeMediaSource implements MediaSource {
       ExoPlayer player, boolean isTopLevelSource, Listener listener) {
     assertThat(preparedSource).isFalse();
     preparedSource = true;
+    releasedSource = false;
     this.listener = listener;
     sourceInfoRefreshHandler = new Handler();
     if (timeline != null) {
@@ -116,6 +117,10 @@ public class FakeMediaSource implements MediaSource {
     assertThat(releasedSource).isFalse();
     assertThat(activeMediaPeriods.isEmpty()).isTrue();
     releasedSource = true;
+    preparedSource = false;
+    sourceInfoRefreshHandler.removeCallbacksAndMessages(null);
+    sourceInfoRefreshHandler = null;
+    listener = null;
   }
 
   /**