Fix visual artifacts in some multi-window scenarios

onDisplayChanged was using ActivityStack's surface bounds
update (ie. cropping) for Task-level.

Bug: 148157819
Bug: 148146791
Bug: 148246658
Test: Enter split-screen, rotate phone, move split around and
      observe that it isn't cropped. Can also use winscope to
      verify

Change-Id: I41d6bb98286d5430d725779cad27ddfa90833fc2