Fix dismiss related issues with bubbles:

- If a bubble is added while the stack is magnetized to the dismiss target,
  it's added in the wrong place.
- If all bubbles are cancelled while the stack is magnetized,
  the stack will reappear at (0,0)

Test: atest SystemUITests
Fixes: 134064858
Change-Id: I7596a471e79190cbce4bbe4541de07783377eed3