SDK / exoplayer

- Avoid re-downloading data prior to the first mdat box when
  seeking back to the start of an unseekable FMP4.
- Avoid re-downloading data prior to the first frame for
  constant bitrate MP3.
- Update SeekMap.getPosition documentation to allow a non-zero
  position for the unseekable case. Note that XingSeeker was
  already returning a non-zero position if unseekable.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177317256