Handle startScoUsingVirtualVoiceCall in HeadsetService

Use case:
Connect headset while in VOIP call

Failure:
SCO is not established with headset

Root cause:
When HS is connected while in VOIP call, sometimes
startScoUsingVirtualVoiceCall() is called even before HSM is
transitioning from Disconnected/Pending to Connected state.

Fix:
Do not ignore startScoUsingVirtualVoiceCall in HeadsetService
if HeadsetStateMachine is still in process of transition to
Connected state.

Change-Id: I8a6c7f3e08b8a9dfc4c4efc6d1d041149874eab0