Support disconnecting conference participants from conference.

- Added "onDisconnectConferenceParticipant" method used to inform a
connection acting as the conference host that one of the participants
should be disconnected.
- Added "VIRTUAL_CALL" PhoneCapability used to indicate that a call cannot
be the active or background call, but only be a child of another call.

Bug: 18228141
Bug: 18176606
Change-Id: I7aec631cc89c0f08e174b24ce3a9cb547b47e36b