Toggle fullscreen state of focused task via fullscreen key

Currently, the fullscreen key is mapped to Ctrl+Action+Up by ag/31495712
and just moves the focused task into fullscreen mode.

However, the key's expected behavior should be to toggle the fullscreen
state of the focused task by moving fullscreen tasks into desktop mode.

With this change, the fullscreen key is mapped (under a flag) to a new
key gesture defined by ag/35177330, which toggles the fullscreen state
of the focused task.

Bug: 396483171
Test: manual
Test: atest InputTests:KeyGestureControllerTests
Flag: com.android.window.flags.toggle_fullscreen_state_via_fullscreen_key

Change-Id: I1d31def5bf01d707c6b01e1f3738d1105c109e6a