screenShield: Activate dialog when necessary

Activating a dialog is slightly different from opening it; the
former is about showing the user authentication widgetry, while
the latter is about creating it and pre-allocating the necessary
resources.

Activate the screen shield dialog when necessary.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/872

js/ui/unlockDialog.js

@@ -527,6 +527,10 @@ var UnlockDialog = GObject.registerClass({
         return true;
     }
 
+    activate() {
+        this._showPrompt();
+    }
+
     popModal(timestamp) {
         if (this._isModal) {
             Main.popModal(this, timestamp);