Add demonstration unittest for TetherInterfaceSM

Prove that the concept works by writing a trivial test.

Bug: 28833951
Test: `runtest frameworks-services -c
  com.android.server.connectivity.tethering.TetherInterfaceSMTest`

Change-Id: I51911cef7f0d5165221845070befd085497c21a6
(cherry picked from commit 1cc6ffd2)