Use fullscreen flag to define visibility of the background

Use fullscreen state provided by AM as a more reliable
source of truth to check if activity window hides other
windows behind.

Bug: 64615361
Test: Open Play Newsstand, click on article, go to and from Recents.
Change-Id: Ibc40a4241ed131f74da47c0d22833d2c3994cbe0
1 file changed