Open SCO for held call.

Use case:
1. Connect to headset/carkit
2. Make an outgoing call from AG
3. Turn off BT on AG
4. Hold the call from AG
5. Turn on BT on AG (No SCO link is established here)
6. Unhold call from AG (still no SCO link is established)

Failure:
No SCO connection established when held call is there
during headset connection.

Root cause:
SCO is not initiated when SLC is done during held call.

Fix:
AG opens SCO connection when HF connected while in call and
the call was in held state.

Change-Id: I63d7655d26ddbf68cbf94c745df745cffa917b5e