Conference calling via HFP

Following scenarios are supported:
1. Accepting incoming call when there is an active call (Call Waiting)
-- We always put the active call on hold.
2. Rejecting incming call when there is an active call (Call Waiting)
3. Merging calls after (1) and going into a conference
4. Ending the conference

Bug: b/29898872
Change-Id: I630bf6d7149fc12eea67f4a49b24fe500f3913d3
(cherry picked from commit 94e2ae2ef4aa4d08855d829251da20585ad977d1)