Expand test coverage of TetherInterfaceSM

Add tests for most normal case operations:
  - State machine startup.
  - Entering a tethered state (as USB interface and not)
  - Handling the first notification of an upstream interface
  - Handle changes in upstream interface
  - Handle unrequested teardown correctly

Bug: 28833951
Test: These tests pass

Change-Id: I2c0c9141bab9eaa92faec8de10fa653fb6236a74
(cherry picked from commit 1b3830a2)