- 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
