Fix crash due to null leash on transition

When dismiss then quick to recents case, divider leash might be
released when dismiss transition finished but recents start
animation later and it will get null leash transition change.

Fix this by add a null check before it being added.

Also remove tricky reparent implementation on finishWCT because
it cause another issue on the above case. We need to find another
more appropriate implementation and animation for dismiss
transition, this will be done on later CL.

Fix: 281605386
Test: manual
Test: pass existing tests
Change-Id: I56d017989482bf0a67c8a80a3569e9fc12c151d9