Implement new external audio route.

1. Track tethered external call in CallAudioManager.
2. Add isTethered as a argument when external state changed for a call.
3. Add TetheredCallState in CallAudioModeStateMachine and relevant
messages to enter/exit this state.
4. Add ExternalState in CallAudioRouteStateMachine and relevant messages
to enter/exit this state.
TODO: change audio mode of AudioManager in CallAudioModeStateMachine to
MODE_EXTERNAL after its implementation completed.

Test: TelecomUnitTest
Change-Id: I3e262a913841706444ab784a1b37ffcda65256ef