SDK / exoplayer

The new behaviour:
- If the session is successfully opened, an acquire event is dispatched
  to all attached dispatchers.
- If acquire() is called but the session is already open, the acquire
  event is dispatched only to the dispatcher provided in to acquire()
- If the session is successfully released, a release event is dispatched
  to all attached dispatchers (in theory at most one should ever be
  attached at this point).
- If release() is called but the session isn't released (because
  referenceCount > 0) then a release event is dispatched only to the
  dispatcher provided to release().

PiperOrigin-RevId: 312062422