Ensure handover calls can be hung up.

It was previously not possible to hang up a call which was in the midst of
SRVCC.  This CL ensures that while the handover is in process we can still
send a hangup request to the ImsCall.

Test: Added new unit text for this case.
Fixes: 152801865
Change-Id: I482f056b62a8a3d7738a20e15ceb44050201464c