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
adae4870fb