Check both TETHER_DUN_APN and TETHER_DUN_REQUIRED to decide DUN APN behavior.
Check both TETHER_DUN_APN and TETHER_DUN_REQUIRED to decide DUN APN behavior.

Bug: b/18158411
Change-Id: Ia1455b67143b968c919d65e9970d7a900eb19044

md5555/android_packages_services_Telephony