Move AbstractFloatingView for stage split layering above all apps

- We don't need to worry about if it's in Overview. Only overview has task menus, so if there is no task menu, we place it at the top.

Fixes: 332465429
Test: Launch stage split using keyboard shortcuts with all apps open. Make sure it's above all apps.
Change-Id: Ic950d2897fe80ef407038dd840b86fc92d6d6287