ScreenShield: place the lock screen contents only on the primary monitor

Use the new monitor constraint to place the clock and notification
box on the primary monitor only. The background is still extended
to the whole screen.
Get rid of the LockDialogGroup hack, now that ClutterBinLayout
respects fixed position correctly.

https://bugzilla.gnome.org/show_bug.cgi?id=681743

js/ui/layout.js

@@ -109,6 +109,7 @@ const LayoutManager = new Lang.Class({
         this.screenShieldGroup = new St.Widget({ name: 'screenShieldGroup',
                                                  visible: false,
                                                  clip_to_allocation: true,
+                                                 layout_manager: new Clutter.BinLayout(),
                                                });
         this.addChrome(this.screenShieldGroup);