SDK / exoplayer

- SimpleExoPlayer now always generates a session ID at
  construction time. This ID is used indefinitely, including
  for tunneling, unless a call to setAudioSessionId is made
  to change it.
- DefaultTrackSelector support for enabling tunneling has
  been changed to a boolean, since tunneling now uses the
  same session ID as non-tunneled mode.
- Since the session ID is now always set at the top level,
  internal propagation of generated session IDs is no longer
  necessary, and so is removed.

PiperOrigin-RevId: 351349687