Expose metered APN types carrier config keys

Public carrier config KEY_CARRIER_METERED_[ROAMING_]APN_TYPES_STRINGS
which has been stable since the initial release in 2016.

With the ability to customize the metered APN type, carriers can improve
user experience by e.g. improving UI or implementing consistent behavior.

Bug: 238634254
Test: atest TelephonyManagerTest#testIsApnMetered
Change-Id: Ia0d7fccfd38156731d491ba3f57c3489f7de8789