IMS-VT: Handle buttons display in InCall Screen

1. If number of incall buttons to be shown are more than the
   threshold, move the add participant button inside more menu

2. Change the order of downgrade to audio and switch camera
   options so that downgrade to audio icon is always visible
   as an icon when in a video call

Change-Id: I43495693906833838f2e0fdf9c328f40708aca12
CRs-Fixed: 959132