sipcall/reinvite: use current media list in re-invite

If multi-stream is disabled and a re-invite is received, the
re-invite answer must use the current media list instead of
creating a new one.
Also, report MediaNegotiationStatus event using the parent
call ID if it's a subcall.

Gitlab: #445

Change-Id: I33cdc5cd4ed8a394d60d8ffb5d506a279fde0b3f