We now tell the system bar every time the top activity has changed for
it to re-evaluate its UI state.
Also fix issue #: 4607102 Low rider notifications. It turns out this
was due to the change in the dialog asset; the notification UI was relying
on this having a lot of padding to make it sit above the status bar.
Now we have an explicitly mechanism to set how much it overlaps (or doesn't)
the status bar.
Change-Id: Iab5ebd86e620ff4fc4cd77206e18af962ec2830e
7d04932ef5