Added support for mid-call enabled SRVCC conference call

When TelephonyConnection is created for an unknown connection,
the recalculation of conference participants have to be
retriggered. This is because, unless that TelephonyConnection is
added to ConnectionService, it cannot be added as part of a
conference call in CallsManager.

CRs-Fixed: 756415
Change-Id: Ic9359104f02eddff655d84fd25f5c28c9158e5f4