Move signal strength logic from ServiceStateTracker to

SignalStrengthController

This is a clean-up CL following introduction of SSC(aosp/1545049).
All changes are logic movement only with no additional function.
Further functional optimization will be in separate CLs.

Bug: 178429976
Test: atest ServiceStateTrackerTest SignalStrengthControllerTest
Change-Id: I53880f03e8f6aa3ddcb43bd9934f0a5f67e62c17