Restrict TaskDisplayArea background color setting to tasks with

fullscreen windowing mode.

Recall: http://recall/clips/3a64e84e-dc27-4a87-a539-2171ad8ba311

Bug: 204253858
Test: Open and close an activity in freeform windowing mode. Ensure that
a blank gray screen does not appear.

Change-Id: I3927d9c6b3f9a72253ef87abcd59325c495f8206
(cherry picked from commit 0ce2d88658d887a4f548e2e05319b3165858bc6c)