Add carrier config to hide merge button for IMS conference

There is the case that operator's network does not support IMS
conference call although it supports IMS normal call. It sometimes
happens when operator starts up IMS service. In such case, it is
necessary to hide merge button for IMS conference.

Bug: 29430010
Change-Id: Iab8afaab297cd401ddea8ef4bbd6a06edfbcfeb3