Expose the carrier signaling broadcasts

Expose broadcasts for communicating certain changes in state between
Telephony and carrier apps.

Implement a backward-compatibility layer to support carrier apps
targeting <=R that are listening to the old hidden broadcasts.

Fixes: 173050428
Test: atest CarrierSignalTest CarrierSignalAgentTest
Change-Id: I15d032ee1e31d9be847c2f084161f8555c8172c1