DesktopMode: Implement method to toggle fullscreen state of focused task

The method will be called when the fullscreen key is pressed, and:
1. Move focused desktop tasks into fullscreen mode.
2. Move focused fullscreen tasks into desktop mode.
3. Move focused split screen tasks into fullscreen mode.

Bug: 396483171
Test: atest WMShellUnitTests:DesktopTasksControllerTest
Flag: com.android.window.flags.toggle_fullscreen_state_via_fullscreen_key

Change-Id: I94db978e27c5560291c6be0d3fbf1c974a54d388