Handover API cleanup.

- Fix issue where empty extras bundle crashes Telecom.
- Complete plumbing of handover complete APIs to Connection and Call.
- Add runtime permission check for acceptHandover.
- Fix bug in acceptHandover where caller identity was not cleared,
resulting in some strange security exceptions.
- Use new EXTRA_IS_HANDOVER_CONNECTION sent to the ConnectionService when
the new APIs are used; this ensures that the CS knows that its a new
fangled handover, and not the legacy API.

Bug: 65415068
Test: CTS tests.
Change-Id: I4b9dafcc0707f1ce58cb6c100588d46fcf0a170f
Merged-In: I4b9dafcc0707f1ce58cb6c100588d46fcf0a170f