Change behavior when resizing docked stack

- Add an API resizeDockedStack to resize the docked stack
and supply temporary task bounds, which can be different from
the stack bounds.
- Use that API in SystemUI to only switch task bounds when
crossing thresholds, so we have less flickering and more
predictable resizing.

Bug: 25015474
Bug: 26311778
Change-Id: Id5c9277dd908ccc28f95dab023efc914757a50d0