Rework audio focus to rely on the native ExoPlayer implementation
instead of a custom implementation.
Previously, we avoided ExoPlayer's AudioFocus system as it never
played after a transient lost. A few versions later now through,
now it does, so we may as well switch to it. This does introduce
a bug where ReplayGain functionality will conflict with audio
focus, but I hope to eliminate this with #115 as I switch to
an AudioProcessor instead of a callback.
Disable the ability to customize audio focus on Android 12 and up.
Android 12 automatically regulates audio streams even further than it
did in previous versions, to the point where the audio focus setting
no longer makes sense on that version. I may extend the removal to all
versions in the future.