SDK / exoplayer

- In IDLE, the button will now call a preparer. This allows
  removal of the separate retry button from the demo app.
- In ENDED, the button will seek back to the default position
  and play.
- Behavior is made consistent with LeanbackPlayerAdapter.

Issue: #3689

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=182506855