SDK / exoplayer

Notes:
- RollingSampleBuffer will be renamed DefaultTrackOutput in a
  following CL, and variable naming will be sanitized.
- TsChunk will also be renamed to HlsMediaChunk, since it can
  be used for non-TS containers (e.g. MP3).
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=120240243