IMS: Send disconnect only for parent calls

- Unable to accept waiting call when waiting call is
  received on top of conference call and user accepted
  waiting call using "answer and release".
- Issue is due to sending multiple HANGUP requests
  for single conference calls, fixing this issue
  by sending HANGUP only for parent call in a
  conference call.

Change-Id: If3c7a996be1510ccc6ed3dfde770d9822db44d7c
CRs-Fixed: 2773919
1 file changed