HeadsetService: Manage voice recognition for multiple devices

* Handle voice recognition in HeadsetService so that we can manage
  multiple connected HFP devices. Implement the following policies to
  manage relations among multiple HFP devices:
  - If voice recognition is initiated by HF, the HF will be set as
    active device and following startVoiceRecognition(device) callback will
    be routed to the initiating HFP regardless of the input argument
  - If startVoiceRecognition(device) is called without a pending request
    from HF, the requested device will be set as active device and audio
    will be routed to that device
* Added unit tests to verify voice recognition function behavior in
  Multi-HFP which can be:
  - Started by HF and stopped by HF
  - Started by HF and stopped by AG
  - Started by AG and stopped by HF
  - Started by AG and stopped by AG
* Fixed logic in HeadsetStateMachine to match the expected behavior

Bug: 35793101
Test: runtest -j40 bluetooth
Change-Id: Ibbabcf7a5eb9355a1f35fb9ff287f7345cdcd372
(cherry picked from commit 2e5b99c2e33a27bed7acfdef46c1e2c95b04f591)