Fix race conditions between Tethering and TetherInterfaceStateMachine

Previously, Tethering would answer requests for current tethering state
by calling methods on instances of TetherInterfaceStateMachine to build
up that state. This is incorrect, since state queries can come in on an
arbitrary thread, independent of updates to the state machines
instances.

Fix this by:
  - Altering TetherInterfaceStateMachine to push consistent state
    snapshots to Tethering via notifyInterfaceStateChange()
  - Storing the last state snapshot in Tethering.mTetherStates
  - Removing public methods to query TetherInterfaceStateMachine state.
  - Consistently synchronizing access to Tethering.mTetherStates

Bug: 29009601
Test: WiFi Tethering continues to work, unittests continue to pass

Change-Id: Ied334f5e8739bc3aff1b08a1079095b9cc2a7958