Enhance logic for determining if answering call will disconnect fg call.

There is logic to determine if a FG video call should be disconnected
when an audio call is received.  It was missing a check to see if VOWIFI
is enabled.  If VOWIFI is not enabled, then the check applies.  If VOWIFI
is enabled, the check does not apply.

Bug: 28933314
Change-Id: I57ace7583906bddb93f81735809a2e21069da380