IMS: Conference URI support.

Add code not to parse dialString in case of
Conference URI and set EXTRAS_IS_CONFERENCE_URI
to ImsCallProfile.
IMS:Null check before retrieving extra from bundle
Add null check before retrieving extras from bundle
to avoid NullPointerException and crash.
IMS: Allow placeCall with complete uri
Add code to dial with complete uri if dial intent contains
extra "org.codeaurora.extra.SKIP_SCHEMA_PARSING" set to true
IMS: Add participant support.
Add ImsPhone's addParticipant() and
ImsPhoneCallTracker's addParticipant()
APIs and send add participant request
with ImsCall.

CRs-Fixed: 1023212
Change-Id: Ia3e0a7cac7bcf9be8bab969be3483cb142186c99