screenShield: Rework key focus management

Instead of always grabbing key focus for the screen lock
group, do that for the unlock dialog itself.

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

js/ui/unlockDialog.js

@@ -418,6 +418,7 @@ var UnlockDialog = GObject.registerClass({
             accessible_role: Atk.Role.WINDOW,
             style_class: 'login-dialog',
             visible: false,
+            can_focus: true,
             reactive: true,
         });