Fix crash when opening overflow after dismissing stack

It changed recently such that we remove the bubble stack
view when its dismissed (previously we just made it
invisible).

This revealed that we weren't cleaning up the bubble
overflow activity view when the stack was dismissed, bc
the parent of that view would be different resulting in
a crash.

This also renamed / simplified a method.

Bug: 157567251
Bug: 155417004
Test: manual - have bubbles, visit the overflow
             - dismiss the stack
             - get new bubble, open it, visit the overflow
             => no crash

Change-Id: Ia2823e5c2cb9b2a53b2379945287588edc30c072