Move signal strength logic from DeviceStateMonitor to

SignalStrengthController

This is a pure clean-up CL after SSC is introduced(aosp/1545049).
No addtional functions are added. The following CLs will locate
the functions updates, including further optimization of cellular
signal strength handling.

Bug: 179077640
Test: atest DeviceStateMonitorTest ServiceStateTrackerTest
SignalStrengthControllerTest

Change-Id: I5512898b2e0e5995ec1fccae1875760f84ae4ea5