Fix fillsParent() always be false on finishing activity

ActivityRecord#occludesParent() returns true only if the activity
is not finish and is opaque since 99d6f0e8, which caused side-effects
to ActivityRecord#fillParent().

Bug: 155343623
Test: Close a freeform activity and examine the app transition
Change-Id: I30f7071e5dadade8dc0aa3a7c8a573bf72c81f72