Adding stock d850 APN file with new ATT Nextgenphone APN.

Use stock AT&T G3 (d850) apns-conf.xml.
CM APN file causes issues with MMS.
Adding new ATT APN causes conflicts with existing APN entries.
Deleting existing APNs in apns-conf.xml may cuase issues with other devices.

Change-Id: I1f0c10476d2082cf502b30fd51d512c807021001