Reset Task surface to no-crop after pip expand when fullscreen

Previously, pip expand was hard-coding the fullscreen surface
to the bounds-at-the-time. It's supposed to be null when in
fullscreen otherwise it won't properly adapt to display changes
like rotation.

Bug: 168495984
Test: Open app in fullscreen portrait, go to pip, expand the pip,
      rotate device and see that it works (no half-black screen)
Change-Id: Ic39cf484438c379009eaf463b22a04f7a86497a3