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.

Bug: 27170551
Change-Id: I9a5ca67402effbf22135addcbd3be240d36669b8