Fix and improve handling of audio session IDs

[master255]

@ojw28 Perhaps you need to pay special attention to AudiosessionId, and to test its work properly?
I regularly come across the fact that it does not work:
#6954.
#6975

ExoPlayer 2.12.0
Android 4.4 audio session stops transmitting data after rewinding and when you switch to the next track. There are no errors or notifications in the logs.

mMediaPlayer.getAudioSessionId() - method does not work. It regularly returns 0 at the time when the session number is different.

[ojw28]

If you want us to investigate this, please provide proper reproduction steps, ideally in the form of a minimal test app. Note that #6954 also lacked clear reproduction steps, to the extent that it's still unclear whether there was a bug. #6975 also lacked clear reproduction steps until 12 messages in, and when they were finally provided we were able to provide a clear answer.

[ojw28]

Handling of audio session IDs is much improved in a95b2eb, which will be in 2.13.0.