SDK / exoplayer

- This change fixes seeking before the prepare (or more
  accurately, before the timeline is set). The fix a minimal
  one to fix the behavior. It's inefficient compared to
  posting the pending seek onto the playback thread, which
  will be the long term solution.
- As of this change, I think we can call V2 "done". There are
  some loose ends to tie up, but the API is effectively
  finalized and the implementation is in a state where you
  can take it, use it and expect it to work.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=132468107