SDK / exoplayer

These were caused by two issues:
1. The FakeMediaSource can be updated with a new timeline. The setNewSourceInfo
is called from a different thread than prepareSource and both access local
variables without synchronization.
2. For multi-window playback, the FakeRenderer claims that isReady and isEnded
are both set to false if it read the end of the stream. However isReady should be
true because it is able to "render" its data until the end of the stream.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=182785169