Reordering the app bubble root task when needed

Make the bubble root task always-on-top while any bubble expands.
Reorder the bubble root task to bottom when all bubbles collapse.

Bug: 407669465
Test: BubbleControllerTest
Test: BubbleUtilsTest
Test: start an app into bubble
Flag: com.android.window.flags.root_task_for_bubble
Change-Id: Idf7c168a1e78579d162123870e00c55b56aed982