greeter: Lower layershell layer to Top

We have a wayland protocol for requesting windows to be raised over the lockscreen. However, the greeter's layershell layer is Overlay, which makes it above other windows (though this doesn't seem to be consistent, seeing as the phone app works fine?). Lower the layershell layer of the greeter so that other windows can be drawn over it.

Needed for plasma-mobile!270 (merged)