Add getSystemService(Class<T>) to align the capability with framework
part.
This is a back port from aosp/1639943, aosp/1645152 and aosp/1648047
Bug: 179640862
Test: local
Change-Id: I035db55a71f94000ca35f8d71f03c19208423c73
Connectivity is becoming a mainline module in S, so
ConnectivityManager hidden symbols can not be used for outside
components. Besides, most Tethering relevant methods or variables
on CM are migrated to TetheringManager. So replace all these
methods or variables from ConnectivityManager to TetheringManager
on Settings.
Bug: 180693313
Test: make RunSettingsRoboTests ROBOTEST_FILTER=<Modified Test>
Change-Id: Iba4b121a4ddf3f04252aa0355e0e6494a593682a