SDK / exoplayer

A session may not notify its release back to the controller
when the controller is connecting while the session is releasing.
Here are issues in detail:
  - MediaSession doesn't respond to controller's connection
    request when it's released.
  - MediaSession discards incoming connection requests when it's
    released. The requests are only kept in the application
    looper, but the looper is cleared when the session is
    released.

This CL fixes the above issues, and fixes some flaky tests.

PiperOrigin-RevId: 419756158