Fixed bug with activity behind non-fullscreen activity not displaying

Regression introduced in I5bf77063252a5abae4e327f6fc42e607091749f9
where some policy for visiblity of tasks in freeform stacks was
added which ended up applying to other types of stacks. We now limit
the policy to just freeform tasks.

Bug: 23957523
Change-Id: Ib6fad5a89e22d64adf0e52b9e8ed5f2239488245