ScreenShield: hide Removable devices in the lock screen

Showing the removable devices is potentially a security risk (as
they include network shares). Also, a nautilus launched from there
can't be used, so it's just a way to overload the system.

https://bugzilla.gnome.org/show_bug.cgi?id=681143

js/ui/messageTray.js

@@ -1037,6 +1037,7 @@ const Source = new Lang.Class({
         this.isTransient = false;
         this.isChat = false;
         this.isMuted = false;
+        this.showInLockScreen = true;
 
         this.notifications = [];
     },