Make LTE signal bar thresholds customizable with carrier configuration

Set the threshold of LTE signal bar to SignalStrength.

Test: manual - Checked that the LTE signal bar thresholds can be
controlled by carrier config.
Test: auto - Confirmed that run
ServiceStateTrackerTest#testSignalStrength and it has no problem.
Bug: 70698348
Change-Id: I65ac32944903193b2a93d3850a02d588624ee9fb
Merged-In: I65ac32944903193b2a93d3850a02d588624ee9fb