The 'player commands' returned to ExoPlayerImpl instruct the
player on how to treat the current audio focus state.
The current return value when playWhenReady==false is misleading
because it implies we are definitely not allowed to play as if
we've lost focus. Instead, we should return the actual player
command corresponding to the focus state we are in.
This has no practical effect in ExoPlayerImpl as we already
ignore the 'player command' completely when playWhenReady=false.
To facilitate this change, we also introduce a new internal
state for FOCUS_NOT_REQUESTED to distinguish it from the state
in which we lost focus.
#cherrypick
PiperOrigin-RevId: 644416586
(cherry picked from commit 66c19390e2b31106a2a467366f7945c37a273fc6)
bd5f99cb09