Pinned stack animation: Correct scaling calculations.

Previously we were scaling to the clip rectangle which included the Surface
insets. When we changed to scaling to the stack bounds, we didn't update
this code, and reversed the insets in a situation they weren't applied.

Bug: 35396882
Test: Move skeleton pip app from fullscreen to pip, ensure it reaches correct final size without jump at end.
Change-Id: If2dc272f40f90383e35d509f7220c21e0e0be5b1