IMS: Conference URI support.

Add Telephony Property PROPERTY_DBG_IMS_VOLTE_ENABLE,
extras EXTRAS_IS_CONFERENCE_URI, EXTRA_DIAL_CONFERENCE_URI
which will require by Telecomm, TeleService, Telephony
Framework, IMS framework etc.

CRs-Fixed: 713839
Change-Id: I2040bd040a46f7f416b3689cf87b351530f8e293