Ensure dismiss-splitscreen makes an app fullscreen

This CL ensures that the top app leaves splitscreen to be fullscreen if
the splitscreen is deactivated unless it’s explicitly moved to desktop windowing.
If the display windowing mode is freeform, the app windowing mode needs
to be set explicitly to be fullscreen.

Bug: 364927895
Bug: 384840232
Flag: EXEMPT bug fix
Test: StageCoodinatorTests
Change-Id: Ia8b6ee332baa486fc598b4b9cd120257aa11e009