Allow split apps to use fullscreen button.

When the fullscreen button is clicked in a split app's handle menu,
dismiss the other app and move the chosen app to fullscreen.

Additionally, set the windowing mode to WINDOWING_MODE_UNDEFINED rather
than WINDOWING_MODE_MULTI_WINDOW as the latter interferes with the
existing split screen -> fullscreen framework.

Bug: 306230346
Test: Manual
Change-Id: I98605c53c5aa7ef7d471ff12aed1772a84448075