[mobile] split out CarrierConfigRepository interface for testing

Makes it easier to mock out in Kosmos. Then I went ahead and added it to
kosmos.

Bug: 364360986
Flag: EXEMPT refactor
Test: CarrierConfigRepositoryImplTest
Test: MobileConnectionsRepositoryTest
Test: all other sysui tests
Change-Id: I86caeb60e9036a202eced6f05ec69ad104c6aa45