Fix dismiss not working in landscape

In stack animation controller only one target would ever
be added so when the target is re-created after rotating
then it wouldn't be used.

Test: - have some bubbles, drag them around, rotate device, try to
        dismiss them => they can be dismissed in landscape
      - get more bubbles, rotate back to portrait, try to dismiss
        them => they can be dismissed in portrait
      - drag the stack around the edges of the screen and verify that
        the only magnetic spot is where the dismiss target is
Fixes: 193008696
Change-Id: I6b24b1f9d301d37749dc67c95392fd6c8451df00