Fix transition offset when closing with reparent

When a window is closing while reparenting to a new window (which is
also closing in this case), the new window will not be included in
the transition. The closing window will be reparented to the transition
root, so it's end offset will no longer be reliable.

Bug: 275034335
Test: verify manually with RTL enabled following repro steps in bug
Change-Id: Iac4a626793119531c60dd4d6753244d47308cee1