Initialize sample streams in FakeAdaptiveMediaPeriod.

This prevents NPE when release or stop is called before
tracks have been selected.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=179911907

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

@@ -44,13 +44,18 @@ public class FakeAdaptiveMediaPeriod extends FakeMediaPeriod
   private ChunkSampleStream<FakeChunkSource>[] sampleStreams;
   private SequenceableLoader sequenceableLoader;
 
-  public FakeAdaptiveMediaPeriod(TrackGroupArray trackGroupArray, EventDispatcher eventDispatcher,
-      Allocator allocator, FakeChunkSource.Factory chunkSourceFactory, long durationUs) {
+  public FakeAdaptiveMediaPeriod(
+      TrackGroupArray trackGroupArray,
+      EventDispatcher eventDispatcher,
+      Allocator allocator,
+      FakeChunkSource.Factory chunkSourceFactory,
+      long durationUs) {
     super(trackGroupArray);
     this.eventDispatcher = eventDispatcher;
     this.allocator = allocator;
     this.chunkSourceFactory = chunkSourceFactory;
     this.durationUs = durationUs;
+    this.sampleStreams = newSampleStreamArray(0);
   }
 
   @Override
@@ -62,13 +67,12 @@ public class FakeAdaptiveMediaPeriod extends FakeMediaPeriod
   }
 
   @Override
-  public void prepare(Callback callback, long positionUs) {
+  public synchronized void prepare(Callback callback, long positionUs) {
     super.prepare(callback, positionUs);
     this.callback = callback;
   }
 
   @Override
-  @SuppressWarnings("unchecked")
   public long selectTracks(TrackSelection[] selections, boolean[] mayRetainStreamFlags,
       SampleStream[] streams, boolean[] streamResetFlags, long positionUs) {
     long returnPositionUs = super.selectTracks(selections, mayRetainStreamFlags, streams,
@@ -79,7 +83,7 @@ public class FakeAdaptiveMediaPeriod extends FakeMediaPeriod
         validStreams.add((ChunkSampleStream<FakeChunkSource>) stream);
       }
     }
-    this.sampleStreams = validStreams.toArray(new ChunkSampleStream[validStreams.size()]);
+    this.sampleStreams = validStreams.toArray(newSampleStreamArray(validStreams.size()));
     this.sequenceableLoader = new CompositeSequenceableLoader(sampleStreams);
     return returnPositionUs;
   }
@@ -131,4 +135,8 @@ public class FakeAdaptiveMediaPeriod extends FakeMediaPeriod
     callback.onContinueLoadingRequested(this);
   }
 
+  @SuppressWarnings("unchecked")
+  private static ChunkSampleStream<FakeChunkSource>[] newSampleStreamArray(int length) {
+    return new ChunkSampleStream[length];
+  }
 }