Send error for if VR is initiated during call.

Use case:
Connect to BT headset.
Make a call. Let the call audio be on BT headset.
Initiate voice recognition(VR) from BT headset.

Failure:
AG is not sending error for VR when initiated during call.
Headset might go into bad state if AG don't send error.

Rootcause:
AG is not sending error for VR when initiated during call.

Fix:
When VR is activated during call, send error response to headset.

Change-Id: Ib12d464fc6a6948ddf22250a65a9e5bfeb4a8fb1