IMS-VT: Use capabilities to show call modify options

- Show upgrade to video receive, transmit or bidirectional
  if video receive, transmit or bidirectional is supported
  in capabilities

- Show downgrade to voice option only when downgrade to voice
  is supported in capabilities

Change-Id: I49d1a609e1ec6432f5dd175c46d5d13b728894f4
CRs-Fixed: 800130