Fix layout and retina'ify Android upsell banner

### Before
![screenshot_2014-06-01-22-48-52](https://cloud.githubusercontent.com/assets/80089/3148955/47cfbe94-ea66-11e3-8f0e-7f18ee1a922d.png)

### After
![google chrome](https://cloud.githubusercontent.com/assets/80089/3148959/4c8a22ee-ea66-11e3-8360-d2c072ea6131.png)