SDK / exoplayer

- Use A/V tracks only for buffering position when available
  in ExtractorMediaPeriod.
- Fix layering of exo_simple_player_view so that subtitles
  are above album art. The test stream provided has both.
- Make album art in SimpleExoPlayer view respect the resize
  mode, like we do for video.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=137698473