SDK / exoplayer

1. Add EventLogger right away in PlayerActivity, else it doesn't log
   playWhenReady being initially set to true.
2. Remove EventLogger logging for the audio position advancing. It's
   redundant with isPlaying logging unless you're very specifically
   interested in the timing difference.
3. Remove unnecessary comment in Player.
4. Fix Timeline Javadoc.

PiperOrigin-RevId: 328983944