HF should wait for AG to change conference

We should always wait for AG to make changes to the call before we show
it to the user so that the UI does not get confusing (inconsistent).
Currently we are not waiting for the conf. call to be
disconnected/established by AG before we show this on our end.

Bug: b/29898872
Change-Id: Ia9198958122e9e539fb6f2a6f08942d3174ae2fc
(cherry picked from commit de8d996c)