[Sb refactor] CarrierConfigRepository

CarrierConfigRepository is meant to replace the old CarrierConfigTracker
class and be easier to add new keys to. It exposes the ability to get a
SystemUiCarrierConfig object which exposes tracked keys as individual
Flows to be consumed in the modern-arch way.

Test: CarrierConfigRepositoryTest
Test: SystemUiCarrierConfigTest
Test: MobileConnectionRepositoryTest
Bug: 238425913
Change-Id: Id3ae744f62aae526bdd8573151632814617c1bd7