This CL adds:
- CallInfo.mExtra and CallInfo.mCurrentCallServiceDescriptor. These
can be used by the selector to perform handoff.
- InCallService.setHandoffEnabled to enform the in-call UI that handoff
is allowed.
- InCallAdapater.handoffCall to initiate handoff.
Bug: 13643568
Change-Id: I94c28b10c0e0a253450f14d31ecdc416d5b44ca4