IMS-VT: Update videoProvider in main thread

Setting video provider in seperate thread is causing timing
issues due to which media signalling is happening on
video provider of closed session. So, process merge complete
indication after video provider is updated to ensure that
media signalling happens on video provider attached with
conference imscallsession.

Change-Id: I9a5ca67402effbf22135addcbd3be240d36669b8
CRs-Fixed: 940142