Don't display hidden activities over home screen.

Fixes jank exposed in 10881705. Specifically background activity
animating up along with translucent activity. Repro steps on manta:

1. From home start Settings.
2. Press home.
3. From home start Downloads (translucent activity that takes 85% of
screen).
4. Observe that as Downloads zooms up the 15% boundary that should be
dimly transparent are showing Settings.

The cause was that there is a finishing activity in the Downloads task
that was used to launch the DownloadsActivity. The existence of that
activity kept the logic from recognizing that the home activity was
behind the DownloadsActivity, not the Settings activity.

This fix descends through all of the activities in a task sitting on
home and makes sure that they only keep home from showing if such
activities are not finishing and visible.

Change-Id: I607afce6b0000b4db634f2ce40a6c37fcee369d7