Improve Android Audio Focus (#1897)

Implement audio focus as per android docs:
https://developer.android.com/guide/topics/media-apps/audio-focus
https://medium.com/androiddevelopers/audio-focus-3-cdc09da9c122

AUDIOFOCUS_LOSS should abandon focus and not try resuming audio, this is done with AUDIOFOCUS_LOSS_TRANSIENT

This fixes at least:
- Audio not being paused after focus being taken by some voip applications
- Content resuming and pausing instantly sporadically (some race condition perhaps) when activity was resumed from background.