Fix toucn unresponsive after half fold in split

After fold animation finished, it will always set null bounds crop
to surface and cause root task surface bounds abnormal.
Fix by ignoring root task surface in StageTaskUnfoldController
because it would not effect animation.

Fix: 211132158
Test: manual
Test: pass existing tests
Change-Id: Iece8f7e7d437bdff1cf9e738df3acc075729b993