[Bubbles] Reduce unnecessary TaskView removal to fix flickers

Frequently adding and removing TaskViews seem to be the source of flickers when quickly switching between bubbles. This CL attempts to reduce unnecessary TaskView removals by (1) defer TaskView removal to after all the switch animations are settled and (2) avoid removing and re-adding of TaskView in prepareExpandedView.

Example of flickers before this change:
A->B->C: A->B animation is waiting for B's visibility. Then B->C animation cancels the previous animation, causing A to be removed. But B->C may be waiting for C's visibility, and this causes no bubble to be visible for a short period.

Bug: 403612574
Test: Manual; atest BubbleBarLayerViewTest
Flag: com.android.wm.shell.enable_bubble_anything
Change-Id: If134f951d00c4a38b956bcec559777dec27c0831