SDK / exoplayer

- Event listener is now at the SampleSource level, since the
  individual ChunkTrackStream instances are created internally.
- Event listener receives events corresponding to loads made
  at the SampleSource level (e.g. manifest fetches) in addition
  to those made by the ChunkTrackStream instances.
- Added ability for FormatEvaluators to propagate arbitrary
  information (as an Object) through to the listeners. This was
  a request from YouTube.
- Added significantly more information to each event, for example
  the DataSpec that defines the load being made is now passed, as
  is timing information.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=124732984