Add animation for desktop to fullscreen.

Change the handler for desktop to fullscreen transition using fullscreen
button to ExitDesktopTaskTransitionHandler. This allows us to reuse the
animation used for going to fullscreen by draging the task to the status
bar.

Test: Manual
Test: atest DesktopTasksControllerTest
Bug: 270394022
Change-Id: I8aadf6b04d6eeb2206c1d25d4eb1b59c7038a41e