Expose Apn Setting Field api's

To support fields in APN Settings as public API:
ApnSetting.Builder#setAlwaysOn()
ApnSetting#isAlwaysOn()
Carriers.ALWAYS_ON
Carriers.MTU_V4
Carriers.MTU_V6
Carriers.USER_VISIBLE
Carriers.USER_EDITABLE

Bug: 307038091
Test: atest CtsTelephonyTestCases
Change-Id: I0996c08d0c23d76bdfa859800b667d8b61f292da