If cellular is the mobile network, tethering should select it as
upstream. If DUN is required, tethering should use DUN as its upstream.
Bug: 153614366
Bug: 150644305
Test: atest CtsTetheringTest
Change-Id: Ief9c745322e8705c0e50185e749a81c5a9d20743
Merged-In: Ief9c745322e8705c0e50185e749a81c5a9d20743