Handle move to fullscreen from split screen from request fullscreen API

The Activity#requestFullscreenMode API sets the windowing mode to fullscreen,
but the shell needs to handle the rest. This change intercepts the
transition in StageCoordinator#handleRequest and adds the changes to
have the trigger task exit split screen and go to the top if it is in
split screen and is in WINDOWING_MODE_FULLSCREEN.

Bug: 315352482
Test: Test using multiwindow fullscreen api test app (ag/20460053)
Change-Id: Ic05e7a49b84147a1f5ea03b880df3d2d14d82d5c