API definitions for Call Handover.

Design doc:
https://docs.google.com/document/d/1qY3oAzjff_4A1ttYb_CGrE_OwTRmXMG_KGsIuPT1ey8/edit#

Bug: 65415068
Test: manual
Change-Id: I98000d01293f3c83697a0d6a95c3c415159ce8ea
4 files changed