AudioService: Do not disconnect SCO started externally

* AudioService should not disconnect Bluetooth SCO if it is started
  externally
* When SCO has already started externally, all SCO connection attempts
  through AudioManager API will fail with SCO_AUDIO_STATE_DISCONNECTED
  intent
* Remove SCO_STATE_DEACTIVATE_EXT_REQ state since AudioService could no
  longer disconnect SCO audio that is started externally
* Change SCO_STATE_* values so that they are consecutive

Bug: 79407565
Test: With Bluetooth HFP device connected:
      Test1: make a outgoing call
      Test2: make VoIP call during cellular call
      Test3: make a VoIP call during another VoIP call

Change-Id: I1e0a10aeb0eac59d4826f9defa595aebd85f534c
(cherry picked from commit 33c3a171)