Fix system crash when ActivityEmbedding process died with pending events

Before, when the organizer binder died, it will trigger TaskFragment
removal that can cause surface placement. Now, we make sure to cleanup
all pending events before removing organized TaskFragments.

Bug: 251583070
Bug: 254322068
Test: atest WmTests:TaskFragmentOrganizerControllerTest
Change-Id: Ifec6f763a7e7819c93cc28d71bd0314f3ac9ac0d