Extract the platform signals handling into settingslib

Mobile platform signals will be shared by SysUI and Settings, but
Settings can not access SysUI code directly. Extract this logic into
settingslib so it can be shared by SysUI and Settings.

Bug: 174582703
Test: atest NetworkControllerSignalTest, Manual test
Change-Id: I990c1dc86fd9a04c4ff083478e45850f3b515547