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.

android-exoplayer/src/main/java/com/brentvatne/exoplayer/ReactExoplayerView.java

@@ -624,6 +624,11 @@ class ReactExoplayerView extends FrameLayout implements
     public void onAudioFocusChange(int focusChange) {
         switch (focusChange) {
             case AudioManager.AUDIOFOCUS_LOSS:
+                eventEmitter.audioFocusChanged(false);
+                pausePlayback();
+                audioManager.abandonAudioFocus(this);
+                break;
+            case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT:
                 eventEmitter.audioFocusChanged(false);
                 break;
             case AudioManager.AUDIOFOCUS_GAIN: