The player may suppress playback when waiting for audio focus even if the
state==Player.READY. There is currently no getter or callback to obtain this
piece of information for UI updates or analytics.
Also, it's a important derived state to know whether the playback position is
advancing. Add isPlaying and the corresponding callback to allow retrieving
this information more easily.
Issue:#6203
PiperOrigin-RevId: 268921721
