Fix flicker when dismissing last bubble in bubble bar

Fixes a flicker in dismiss animation for bubble expanded view when it is
the last bubble in the bubble bar.
Delay removing bubble bar layer view from window manager until the
dismiss animation completes.

Bug: 283991264
Test: manual, have 1 bubble expanded with bubble bar, drag it to dismiss
    view, check there are no visual flickers in the remove animation
Test: manual, check in web-hv that BubbleBarLayerView is removed when:
    - drag collapsed bubble bar to dismiss target
    - expand bubble, drag bubble to dismiss target
    - expand bubble, drag expanded view to dismiss target
Change-Id: I29495672c7587d61e4bd2dd717ed5fe83136e366