SDK / exoplayer

- Use a single constant for unset/unknown times across
  all time bases. Note also that this moves away from
  use of -1 for unset/unknown times in ms, which was a
  bad choice (it might conflict with real times, such
  as a time representing being just behind the start
  of a live window).
- Add a few other unset constants, and use them.
- Fix some hardcoding of -1 where existing constants
  should have been used.

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MOE_MIGRATED_REVID=130775594