SDK / exoplayer

Useful functionality promoted to core library:

1. Management of SurfaceHolder.Callback lifecycle
   promoted to SimpleExoPlayer
2. Ability to determine whether audio/video tracks
   exist but are all unsupported promoted to
   MappingTrackSelector.TrackInfo
3. Read external storage permissions check promoted
   to Util
4. SubtitleView given ability to act directly as a
   TextRenderer.Output to remove layer of indirection
5. SubtitleView given ability to configure itself to
   user's platform wide caption styling
6. KeyCompatibleMediaController promoted to library's
   UI package.

Relocation of boring stuff:

1. ID3 frame logging moved to EventLogger.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=128714230