Cleanup TaskFragment reference on remove

Before, we stored organizer created TaskFragment in
WindowOrganizerController, which would only be removed if there is a
request from client.

Now, we also cleanup when the TaskFragment is removed from the display
for any other reason (such as app close),

Fix: 223499926
Test: atest WmTests:TaskFragmentOrganizerControllerTest
Change-Id: Id7c2124bec3e20e9d2466806e20973ec128c463c