layout: Use monitor index when adding bg managers

Don't assume that this._bgManagers.push() (i.e adding to the end) is always
correct.

On startup we call _createPrimaryBackground which passes in the primary index
which may not be 0.

js/ui/layout.js

@@ -362,7 +362,7 @@ const LayoutManager = new Lang.Class({
                               BackgroundMenu.addBackgroundMenu(bgManager.background.actor);
                           }));
 
-        this._bgManagers.push(bgManager);
+        this._bgManagers[monitorIndex] = bgManager;
 
         return bgManager.background;
     },