Remove bubble bar layer from window manager

This change ensures that the bubble bar layer view is removed from window manager when all bubbles are removed.

There are 4 ways to dismiss the bubble bar:
 - Dismissing the last bubble using the menu
 - Dragging the last bubble to the dismiss target
 - Dragging the expanded view to the dismiss target
 - Dragging the bubble bar to the dismiss target

When dismissing using the menu, the state is that we're expanded, so we need to collapse the expanded view and remove the bubble bar layer view at the end of the animation.

For the other options, as soon as we start dragging the expanded view becomes hidden so we're not expanded, and therefore won't collapse. So removing the layer view is done as part of removing the bubble.

Also -- when we initially set up views or after we switch modes, we no longer eagerly create and add the bubble bar layer view and stack view to window manager.

Fixes: 283792836
Test: Manual -- verified through logs and web-hv that the bubble bar is removed from window manager
      - Add a bubble to bubble bar
      - Drag the bubble bar to dismiss it
      - Verify in web-hv the bubble bar layer view is removed
      - Repeat by dragging the individual bubble to dismiss
      - Repeat by dismissing the bubble using the menu
      - Repeat by dragging the expanded view to dismiss
Change-Id: I637a9ade6cbf0509bb23435291dc2ac919ac2332