Create a root view for bubbles

We now use a single window in window manager for both floating
and bar mode. The root view is added to window manager and the
stack and layer view are added under the root view.

See details in go/bubbles-stack-bar-conversion

Bug: 392893178
Flag: EXEMPT refactor
Test: manual - check stuff still works
       - create first floating bubble
       - dismiss bubble
       - create it again
       - unfold
       - dismiss
       - create first bubble bar bubble
Change-Id: I6f907dc3aafc180e430f01287d4a33c4be5384ea