Expose key to override the SIM country ISO

To support subscriptions that belong to a carrier located
in a country that is different from the MCC country of the
ICC, expose a CarrierConfig override that will override the
Subscription's country ISO.

Bug: 215622384
Test: atest SubscriptionManager#testCountryIso
Change-Id: Ie4cffb23cd3a1d4a2f78094d2cd04cdd08583803