Fixed an bug with the notification backgrounds

Notifications could suddenly have the wrong alpha and have therefore the background shadows shining through. This also cleans up the background updating logic
having a single place where their visibilities are updated.
It also fixes the shadow alpha after dragging down on the lockscreen.

Bug: 27314863
Change-Id: I5cb7937c5d2fc89cb7ef2f00d57d704349fd4f94