autorunManager: Follow Files' notification policy

Builtin chat and network notifications already follow the notification
policy of appropriate applications, it makes sense to do the same for
autorun notifications to give users control over hotplug notifications
in Settings.

js/ui/components/autorunManager.js

@@ -313,6 +313,10 @@ const AutorunSource = new Lang.Class({
 
     getIcon: function() {
         return this.mount.get_icon();
+    },
+
+    _createPolicy: function() {
+        return new MessageTray.NotificationApplicationPolicy('org.gnome.Nautilus');
     }
 });