SDK / exoplayer

- When transitioning to a new period, the value of bufferedDurationUs
  passed to TrackSelection.updateSelectedTrack was incorrectly set to
  0. It should have been set to correctly reflect buffered media in
  previous periods still being played out.
- This change fixes the issue described above, and also propagates the
  playback position through to this method. The position of the next
  load within the period can be calculated by adding the position and
  bufferedDurationUs.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=172736101