Save the stack position on dismiss, so new bubbles appear in the original position.

Also, moves the fling/spring logic into the StackAnimationController. It logically belongs there anyway, and also it makes it easier to save the resting position when the fling/spring animations are complete.

Fixes: 126726949
Test: atest SystemUITests
Change-Id: If788e08d1401016bb1135416d495fbb5abf82cbb