Draft: service-provider: Look for mms apn instead of active bearer

Instead of looking for the active bearer look at the usage attribute for mms for the mmsproxy and mmsc.

We only use that for data lookup but don't change the APN name as nothing outside mmsd-tng sets up that APN atm and mmsd-tng itself doesn't do it either (see #63)

This is a rework of !60 (merged) to enhance autoconfig but leaving the APN itself unchanged.

If this idea flies I'll adjust the tests (looking at the XML all reasonable APNs have type mms set).