Handoff: Add APIs to allow handoff between call services

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
8 files changed