MessageTray: introduce configurable per-source notification policy

Allow message tray sources to provide a NotificationPolicy object,
that will configure how and if the source is displayed. For notification
daemon sources, this object is hooked to GSettings.

https://bugzilla.gnome.org/show_bug.cgi?id=685926

js/ui/components/autorunManager.js

@@ -293,7 +293,6 @@ const AutorunResidentSource = new Lang.Class({
 
     _init: function(manager) {
         this.parent(_("Removable Devices"), 'media-removable');
-        this.showInLockScreen = false;
 
         this._mounts = [];
 
@@ -301,6 +300,10 @@ const AutorunResidentSource = new Lang.Class({
         this._notification = new AutorunResidentNotification(this._manager, this);
     },
 
+    _createPolicy: function() {
+        return new MessageTray.NotificationPolicy({ showInLockScreen: false });
+    },
+
     buildRightClickMenu: function() {
         return null;
     },