Adding Carrier Config API.

UICC privileged carrier apps will extend CarrierConfigService to provide
carrier-specific configuration. Apps/services will use
CarrierConfigManager to read the current configuration.

CarrierConfigManager also defines the set of configuration variables and
their default values.

Bug: b/19483786
Change-Id: I027211b43276afd6fe893ae50048c52f2aed5cf5