messageTray: Take over trayBox from LayoutManager

messageTray: Take over trayBox from LayoutManager

Since commit e189a34, the trayBox uses a Contraint to cover the primary
monitor's work area. This allows banners to be clipped so they don't
leak into monitors above the primary one during animations. However even
without being reactive, the trayBox now interferes with operations like
Looking Glass' object picker and overview DND.
With the trayBox no longer being positioned manually, there's no strong
reason to keep it in LayoutManager, and handling it in MessageTray allows
to hide the actor while no banner is showing, which helps with the issue
outlined above.

https://bugzilla.gnome.org/show_bug.cgi?id=744912