Expose ApnSetting#TYPE_*_STRING constants

Expose these string representations of APN types as SystemApi, and add
utility methods to convert between these and their integer equivalents.

Fixes: 173133122
Test: atest ApnSettingTest TelephonyRegistryTest
Change-Id: I8095ebc4f766be52042eec1c8ee043bf2e2dabfb
Merged-In: I8095ebc4f766be52042eec1c8ee043bf2e2dabfb