top-bar: Lower top bar for fullscreen and system modal dialogs
Follow up from !1053 (comment 1420407)
We currently move the top-bar to the overlay layer by default. This way it currently is still visible even when there are full screen views. Also system modal dialogs don't stretch over it.
Fix is to keep the top-bar on the top layer and only raise it when shell is locked. That needs phoc changes though as the the lowering the top-bar back to the top layer changes surface order and hence the overview stretches over it.