Defer removal of activity until animation complete.

Do not remove an AppWindowToken or its Task if it is running an
animation. Doing so leaves windows orphaned and bad things happen
when the system tries to retrieve the task assoicated with the
windows during layout.

Fixes the ugly output associated with bug 13440426.

Change-Id: I9d5fc50a7cb6189160d51cee3c82f7813baee715