Fix DUN based Tethering.

Turns out there was a case issue where the DUN commonly found in the tethering
apn config (in config.xml) didn't match the dun in PhoneConstants.APN_TYPE_DUN
so the NetworkCapabilities generating code din't mark it as DUN capable.

Fixed in the APNSetting as this is constructed infrequently.

bug:16242255
Change-Id: I90504e36bf58ee9b4eb1a757e7daf4786d47a875