Don't use SysUI ScrimController for bubbles

Changes for the manage menu will require a scrim between the expanded
view and the manage menu.

I think it makes sense to just have BubbleStackView control the
scrims entirely rather than have 1 in ScrimController and 1 in
BubbleStackView (or 2 scrims in ScrimController).

Test: manual - expand a bubble, there's a scrim behind it, collapse
      the bubble, the scrim is gone.
Bug: 183658858
Change-Id: I537c910b959ab938eb1d116d9dc0c801006dbb79