autorunManager: Mark the Removable Devices source as resident

This ensures that it can't be removed when we clear the message
tray.

https://bugzilla.gnome.org/show_bug.cgi?id=696199

js/ui/components/autorunManager.js

@@ -292,6 +292,7 @@ const AutorunResidentSource = new Lang.Class({
 
     _init: function(manager) {
         this.parent(_("Removable Devices"), 'media-removable');
+        this.resident = true;
 
         this._mounts = [];