Add FF OS tracking onto platform var

We are going to be running a test to see how much traffic will come from Firefox OS.
The tricky bit is knowing if someone is coming from the browser in FF OS or the app ([they have the same user agent string](https://developer.mozilla.org/en-US/docs/Web/HTTP/Gecko_user_agent_string_reference)).

After talking to @rich-nguyen, the apps team, and the Omniture team, this seemed like the most sensible approach.

![Making tracks](http://28.media.tumblr.com/tumblr_lzz4ew0O3R1r9xjxoo1_500.gif)
For some reason I didn't think so, but now I do.
I think that might be more failsafe too though as the default will be `frontend`. Coming soon.