Add automatic audio focus handling to SimpleExoPlayer. Audio focus
handling is an opt-in feature that can be requested by passing
the system's AudioManager and an AudioFocusConfiguration to
SimpleExoPlayer.setAudioFocusConfiguration.
When audio focus is being managed by SimpleExoPlayer, the player
will transparently handle pausing playback during
AUDIOFOCUS_LOSS_TRANSIENT, as well as lowering playback volume
during AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=208045732
9db0dbc0e9