Ensure pinned animation scaling is consistently applied.

For the pinned stack animation, we have the special mode
where setSurfaceBoundaries computes additional scaling
factors to force the window to occupy the stack size
(which we animate). We need to make sure prepareSurfaceLocked
also respects these scaling factors or we have issues when
switching in and out of the fullscreen stack.

Bug: 28899837
Change-Id: I72ccba54b38993693ff6771882fb99ef82af5827