Add handover permission, fill in some missing API gaps.

Adding the ACCEPT_HANDOVER runtime permission which an app must have in
order to accept handovers (this is per design).

Adding missing onHandoverComplete method in the android.telecom.Connection
API (per design).

Finishing plumbing for android.telecom.Call#onHandoverComplete API.

Fix issue where the new handover API methods would never get called; the
legacy handover extra was being used in this case when it should not have
been.

Bug: 65415068
Test: Verified using new CTS tests
Change-Id: If1558f6a23911862c02ac5b18fb62d86911ed7e2
Merged-In: If1558f6a23911862c02ac5b18fb62d86911ed7e2