Fix NPE on WindowContainer#isOnTop

RecentsAnimationController#removeTask will use WC#isOnTop
to check if can accept the remote animation task target removal.

However, it may hit the timing that the parent of the task might be
removed prior than removeTask called, so caued exception during
WC#getParent().

Add null check in WC#inOnTop to return false when WC#getParent is null.

Bug: 158937083
Test: atest WindowContainerTests
Change-Id: I17b1f709722f573af6efcd00523d434766bca800