SDK / exoplayer

- MediaDrmEventListener.onEvent is typically called on the
  app's main thread. mediaDrmHandler is instantiated on the
  playback thread. Hence mediaDrmHandler should be volatile
  to ensure visibility.
- Nulling mediaDrmHandler could result in a NPE in onEvent.
  Instantiate mediaDrmHandler (and don't null it again) to
  avoid this. MediaDrmHandler.handleMessage will correctly
  discard any events for sessions that are now closed.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=168707938