Override dev/device parameter with the query string (bug 1060062)

https://bugzilla.mozilla.org/show_bug.cgi?id=1060062

This will allow /discovery to execute a search for desktop apps only while not affecting anything else (since the UX to allow the user to change how their searches are filtered is still in flux)
I had forgotten about this rewriting, that'll definitely break, I'll fix that.

I also feel weird about the hardcoding, but really this is just a hack before implementing bug 969244.