SDK / exoplayer

- TrackSelection now exposes the selected formats, ordered
  by decreasing bandwidth. This removes the need for DASH,
  SS and HLS to do the sorting individually.
- The change also removes the need to reconstruct TrackSelection
  instances with a different group index in various places
  (e.g. MergingMediaPeriod).
- This is also a step toward potentially packaging the
  FormatEvaluator inside of the TrackSelection (TBD).

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=128159064