[Floaty] Fix Floaty window addition logic

This CL contains 3 bugfixes:
1. Top-UI refactor was not respected for the setRequestTopUi(true) call
2. The addWindow function is changed to be uninterruptible
   Previously, it could happen that it was interrupted during the
   requestTopUi suspending function. This would leave the class in a
   state of root being initialized but not added to WindowManager.
3. root cannot be reset to null anymore without windowManager.removeView
   being called. This could have led to a bug of multiple root views
   being added without them being removed again.

Bug: 414714786
Flag: com.android.systemui.shared.enable_lpp_assist_invocation_effect
Test: TopLevelWindowEffectsTest
Change-Id: I634f21498ce55808e6ed4930f5cea34867347775