BT: Handle startScoUsingVirtualVoiceCall in HeadsetService

When HS is connected while in VOIP call, sometimes
startScoUsingVirtualVoiceCall() is called even before HSM
is transitioning from Disconnected/Pending to Connected state.
Do not ignore startScoUsingVirtualVoiceCall in HeadsetService
if HeadsetStateMachine is still in process of transition to
Connected state.

CRs-Fixed: 725858
Change-Id: I7b0216a9418ed7cba7f7b3af18a9749262d8eee6