Fix NPE on TaskFragment

We need to deregister the WindowContainerListener at an
earlier stage to avoid a translucent activity to have a
reference to an opaque one without a parent TaskFragment
which causes getTaskFragment() to return null.

Fixes: 278899220
Test: atest WmTests:SizeCompatTests

Change-Id: I06bef5f386cffdc7e89e7e39bd33b1eb3c0ff6b8