Bug 828237 - [Keyboard] Switch to new keyboard layout if language is changed. r=djf

Based on the current selected language from the Settings app: add the new keyboard layout group
to the user's enabled keyboards and switch to the keyboard associated with the language
(i.e. `keyboard.current` setting).
Merge pull request #9073 from mcirlanaru/bug_828237

Bug 828237 - [Keyboard] Switch to new keyboard layout if language is cha... r=@davidflanagan