Changed dconf path for MMS settings

Previously, MMS settings were stored right in the IMSI directory at the root of the dconf namespace. They have been moved to /imsi/ folder and MMS settings now live in the mms subfolder:

/IMSI/ -> /imsi/IMSI/mms/

Old settings are migrated to the new location the first time they are requested. Instead of GSettings we now directly use DConf because it appears to be impossible to properly implement migration with GSettings. That makes gschema obsolete but we need to keep it around for backward compatibility reasons.