SDK / exoplayer

- This change snaps the seek position for constant bitrate MP3s
  to the nearest frame boundary, avoiding the need to skip one
  byte at a time to re-synchronize (this may still happen if the
  MP3 does not really have fixed size frames).
- Tweaked both ConstantBitrateSeeker and WavHeader to ensure the
  returned positions are valid.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177441798