Implement /firefox/android page redesign. Bug 1072953.

- Update fx nav. Bug 1073237.
Yeah, the check in general is for a pretty small subset of users... My main concern is with browsers that do support CSS media queries, do not support `matchMedia`, and have a screen larger than 760. If they loaded the page with a skinny browser and resize, the phone slideshow breaks. Was thinking that if we check `screen.width`, we can have a (reasonable?) guarantee that they'll never get wider than 760.