IMS-VT: Use correct video state when accepting the request

To accept session modification request UI needs to reply
with the video state provided in the incoming session
modification request. Current implemenation accepts all
incoming session modification requests by replying with
hard coded bidirectional video state which fails at
the lower layers.

BUG=28917534
Change-Id: If3e51bcc1b7a8a122cffaf28d524c3df24b1c1dd