SDK / exoplayer

- Use FakeDataSource as the upstream source.
- Actually validate that caching is happening (i.e. reads happen
  on the upstream source only if the data hasn't been read through
  the CacheDataSource already).
- Move FakeClock to sit alongside the other Fake classes.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=118555903