Fix split screen exit bug on a freeform task.

Clear the windowing mode of the task that is partially shown on the side
stage so that when the split screen is cancelled, the task is launched
in full screen.

Test: atest StageCoordinatorTests
Flag: com.android.window.flags.enable_full_screen_window_on_removing_split_screen_stage_bugfix
Fixes: 372791604
Change-Id: Ie7c261e274fb0c29595e00e209997610b7ffec77