Support for delayed focus gain

### Use case description
When a media app requests audio focus, it might not be able to get it right away because the focus is "locked" by another app (_e.g._, when a music streaming app requests audio focus during a phone call). Such use cases can be handled within Android through the use of delayed focus gain (see https://developer.android.com/guide/topics/media-apps/audio-focus#delayed-focus-gain). However, as far as I can tell, ExoPlayer doesn't support that feature yet.

### Proposed solution
Add support for delayed focus gain - https://developer.android.com/guide/topics/media-apps/audio-focus#delayed-focus-gain.

### Alternatives considered
- There are workaround-ish ways to handle these use cases (_e.g._, requesting audio focus multiple times) but they are cumbersome and error-prone.
- A more comprehensive option would be to support greater customisation of the SimpleExoPlayer/ExoPlayerImpl classes. A case in point: allowing users to slightly customise the AudioFocusManager used by the player.