System: update lock screen at construction

Will make sure that change notifications are enabled by reading
the value at least once

https://bugzilla.gnome.org/show_bug.cgi?id=746509

js/ui/status/system.js

@@ -125,6 +125,7 @@ const Indicator = new Lang.Class({
                                 Lang.bind(this, this._updateMultiUser));
         this._updateSwitchUser();
         this._updateMultiUser();
+        this._updateLockScreen();
 
         // Whether shutdown is available or not depends on both lockdown
         // settings (disable-log-out) and Polkit policy - the latter doesn't