Clean up closing apps list when clearing anAppWindowToken's task.

Previously it was possible for an AppWindowToken to be removed while
on the closing apps list, used in transition animations. During these
transitions, the visibility of the token is modified. Since
visibility relies on the WindowContainer parent, a
NullPointerException would occur.

This changelist addresses the issue by making sure to remove any
AppWindowToken from this list when its task is set to null.

Change-Id: Id9234822b228f4658f04d42ac0fe7b49ded6f5a1
Fixes: 35352214
Test: manual (primarily code inspection)