Make string version of APN_TYPEs a @SystemApi

The string version of Apn Types are used in the carrier config table.
Modules besides Telephony uses this table directly and so it made sense to expose
this as a @SystemApi

Test: ApnSettingsTest
Bug: 147465477
Merged-In: I1c344b3ff482d973b8bf51a33956a7f90f291634
Change-Id: I1c344b3ff482d973b8bf51a33956a7f90f291634
3 files changed