Fix Bug 904592, enable l10n for FHR for Android

@lauraxt r?

So, I do not know if my Python is as Pythonic as it can be and I need some feedback also from Pascal regarding the l10n hooks but, all in all, it works.

The generate script now takes a new parameter called --platform that can either be desktop or mobile. If you do not pass anything, it defaults to desktop. Also, if you do not specify a output-dir it will write to a build directory at the root of the repo as such:

    /build
      /html

and

    /build
      /mobile

respectively.