Introduces class SignalStrengthController

SignalStrengthController is the dedicated handler of signal strength
logic. The follow CLs will move logic from SST/DSM to SSC.

To avoid creating an useless skeleton, as the first step, move all
logic of triggered signal strength reporting feature from SST to SSC.

The is purely refactoring CL, with no new functionality added.

Bug: 178429976
Test: atest ServiceStateTrackerTest SignalStrengthControllerTest
Test: triggered signal strength reporting feature test
Change-Id: Id01276aa2b0551239f3d42a64ac07d665aaa7705