IMS-VT: Add call modifiable capability

PhoneCapababilities call type modifiable constant added

Change-Id: I548b3644e61ec1807f8f79f0dec6e12ab8de6eac
CRs-Fixed: 746297
IMS: Clean up Conference URI system property.

Remove system property "persist.dbg.ims_volte_enable".
Add ADD_PARTICIPANT in PhoneCapabilities.toString().

Change-Id: Ic9410ff65319a52d7127c47885e1ce56805fe47a
CRs-Fixed: 727689