SDK / exoplayer

- Remove skipping of the VBRI/XING frame before calculating position
  offsets. This was incorrect. Instead, a constraint is used to ensure
  we don't return positions within these frames, the difference being
  that the constraint adjusts only positions that would fall within
  the frames, where-as the previous approach shifted positions through
  the whole stream.
- Excluded last entry in the VBRI table because it has an invalid
  position (the length of the stream).
- Give variables in XingSeeker descriptive names.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177451295