apns: add "phone" APN for AT&T

  * These "phone" provisioned SIM's don't work with the "pta" APN
  * LTE and MMS do work properly with "phone" APN

Change-Id: I782f9ee5c642c5354ab35489a9c899a52c0d5107
Merge remote-tracking branch 'upstream/cm-10.1' into HEAD