Wire floating bubble to bar transition

When unfolding with an expanded floating bubble, we now convert it
to bar using shell transitions.

Also make sure that the overflow is initialized correctly for bar
so that it can be displayed.

Bug: 392893178
Flag: com.android.wm.shell.enable_bubble_bar
Test: atest BubbleControllerTest
Test: atest BubbleBarExpandedViewTest
Test: manual
       - create floating bubble and expand
       - unfold
       - observe bubble is expanded correctly
       - repeat with overflow expanded
Change-Id: I07ab58264db1f08a668dde183dc370c9d6aac06f