1022767 use json l10n manifests

https://bugzilla.mozilla.org/show_bug.cgi?id=1022767
This is an intended change. Without it, isSubjectToDeviceType works for path like this:
 - /locales/fr/browser/device_type
but does not work for:
 - /locales/fr/browser/device_type/phone/
 - /locales/fr/browser/device_type/phone/phone.en-US.properties
Which means that there has to be additional work put to deconstruct path to make it work with the function.
I believe that the change makes the utils function more usable as it correctly evaluates any url that uses device_type in the path.

I plan to file a follow up to make isSubjectToBranding do the same.

If Yuren will oppose this change, I'll rework the part of multilocale that wants to use it, but would prefer to try to extend the functionality of this function first. Does it sound good to you?