Bug 886757 - We need to load the operator size limit for MMS from the apn.json file - r=kaze

* Loading operatorSizeLimitation from apn.json on startup
* Tweak apn.json creator to load setting from operator_variant.xml
* Add values from TEF for operator_variant.xml
(cherry picked from commit 151a1a160d22236a4664440c2f504364ec4a8891)