Reparent SurfacePackage to null when releasing it

Previously, before BLAST, we were relying on the recreation of the
SurfaceControl when the SurfaceView was detached and reattached to
remove the SurfacePackage from the display. With BLAST we
can continue to reuse the existing SurfaceControl so this fix explicitly
removes SurfacePackage offscreen before releasing it.

Fixes: 180436742
Test: try bug steps (Change grid options in wallpaper settings and check for no overlap)
Test: go/wm-smoke
Change-Id: I9ffbc4b4a724333c799cb57f67c1fc34fe94ce41