Move some logic to MobileState

Some variables were being created in
MobileSignalController#notifyListeners() that were just an aggregation
of a few fields on MobileState. This CL replaces those with method
calls on MobileState to clean things up and make them loggable from the
state.

Test: atest SystemUITests
Test: m -j RunSettingsLibRoboTests ROBOTEST_FILTER="com.android.settingslib.MobileStateTest"
Bug: 197851948
Change-Id: I06d5e56923897cddb53189359dc9444cdfee6e78