SDK / exoplayer

The TrackSelector API will look like:

TrackSelection[] selectTrack(
    TrackRenderer[] renderers, TrackGroup[] trackGroups);

In this CL:

- SampleSources return TrackGroup[], so that the result can be easily
  passed to the selector.
- TrackStream gets its own file to sit alongside other Track* classes.
- A TrackSelection object is introduced to encapsulate group and track
  indices.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=115251331