Fix resting stack position + dot position issues.

The resting stack position issue was due to the position being set when the 'following' spring animations end. When there's one bubble, there's no following animations so this was not called. This is fixed by setting it in the fling animation end listener instead, which is called when the first bubble's fling ends.

The dot position just needed to be set when the first child is added.

Test: atest SystemUITests
Fixes: 138301618
Fixes: 134064116
Change-Id: I5360f09bfda95a45ada534101cb058c26f7c37f5