Fix visibility of multiple non-fullscreen activities.

Issue detail:
Assume X, Y are non-fullscreen activities.
 a.Home starts an activity X in task A in application stack.
 b.X starts an activity Y in <task A> or <new task B>
 c.Activity X will be invisible.

How to fix:
Because the function "isActivityOverHome" means an activity is able to see home.
But there may have many non-fullscreen activities between the top non-fullscreen activity and home.
If flag "behindFullscreen" is set, those middle activities will be invisible.
So it should only take care from who is adjacent to home.
Then check two flags frontOfTask(task root) and mOnTopOfHome for constraining the condition.

Change-Id: I3543fce57327273a9e6f95ad29d98fc3c04eb5bb