Do not accept SCO without any active listener.

Use case:
1. Connect to Motorola s305 headset.
2. start playing music on Phone.
3. Make an MO call for last dialled number from headset.
4. Accept and end the call from remote phone.

Failure:
After call ends, music resumes on headset for 1-2secs and
routed to speaker.

Root cause:
Headset is re-connecting SCO after call disconnection.

Fix:
 - Some HS re-connect SCO after call disconnection.
   Do not accept such incoming SCO connection requests.
 - Check for valid SCO listener(active CS or VOIP call, VR started)
   before accepting incoming SCO connection requests.

Change-Id: Ifcb1f760b3eabf52d9b507c5d25021b90bfec561