Fix bug in FakeChunkSource.

In order to retrieve the data set, the track selection index was used, but
the data set is actually indexed by track group indices.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=167566049

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

@@ -92,7 +92,8 @@ public final class FakeChunkSource implements ChunkSource {
       Format selectedFormat = trackSelection.getSelectedFormat();
       long startTimeUs = dataSet.getStartTime(chunkIndex);
       long endTimeUs = startTimeUs + dataSet.getChunkDuration(chunkIndex);
-      String uri = dataSet.getUri(trackSelection.getSelectedIndex());
+      int trackGroupIndex = trackSelection.getIndexInTrackGroup(trackSelection.getSelectedIndex());
+      String uri = dataSet.getUri(trackGroupIndex);
       Segment fakeDataChunk = dataSet.getData(uri).getSegments().get(chunkIndex);
       DataSpec dataSpec = new DataSpec(Uri.parse(uri), fakeDataChunk.byteOffset,
           fakeDataChunk.length, null);