op shelf landing changes for tablets (bug 1067432)

With 1 app:
![](http://f.cl.ly/items/0F0P2e2O0J410E1R091s/Image%202014-09-16%20at%201.56.21%20PM.png)

With > 1 app:
![](http://f.cl.ly/items/371k1k3O2V0T1q41041l/Image%202014-09-16%20at%202.05.51%20PM.png)
Maybe we should fallback to background_image if background_image_landing doesn't exist (for old shelves).