Bug 958893 - Pre: Add FxAccountUtils.unwrapkB.
Bug 958893 - Part 1: Round out FxAccountClientRemoteException.
Bug 958893 - Part 2: Add and use FxAccountLoginStateMachine.
Bug 958893 - Part 3: Propagate token server errors back to state machine.
Bug 958893 - Part 4: Add logging to Cohabiting and Married states.
Bug 958893 - Part 5: Add debug button that forgets certificate.