Make TetheringManager to system API

Also deprecated tethering APIs in ConnectivityManager.
Will have follow up change to remove @hide tethering function in
ConnectivityManager.

Bug: 145093446
Bug: 148038547
Test: -build, flash, boot
      -atest TetheringTests

Change-Id: Ia432057bf9056727c4a0ca97d160a49274d33581
Merged-In: Ia432057bf9056727c4a0ca97d160a49274d33581