Define new key gesture to toggle fullscreen state of focused task

The new key gesture KEY_GESTURE_TYPE_TOGGLE_FULLSCREEN toggles the
focused task's fullscreen state. See ag/35049859 for the actual logic to
toggle the focused task's fullscreen state in DesktopTasksController.

The key gesture will be mapped from the fullscreen key (under a flag) by
ag/35049860.

Bug: 396483171
Test: atest WmTests:KeyGestureEventTests
Test: atest WMShellUnitTests:DesktopModeKeyGestureHandlerTest
Flag: com.android.window.flags.toggle_fullscreen_state_via_fullscreen_key

Change-Id: I759ee7c95a42d701c55d7cd8ddfc25976d44cd94