Listen to carrier config change in SignalStrengthController

Instead of depending on ServiceStateTracker to listen
and forward CC change event, SSC now handles by itself.
This not only removes the dependency but also be able to
cache the carrier config and only query when change occurs.

Bug: 178429976
Test: atest SignalStrengthControllerTest
Change-Id: I78394ce658400eb428d687214af379e0c961e0de