SDK / exoplayer

- This avoids the need to have to use the timeline to
  calculate a window duration, which can be awkward.
- Window now represents a window of availability with
  an isSeekable flag, rather than a window of
  seekability.
- Promoted Timeline and Window to top package; they're
  pretty important :).

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=130278509