API Documentation and Constant cleanup.

1. Update handover API docs for clarity.
2. Added an unknown value per API review comments.
3. Renamed HANDOVER_FAILURE_DEST_USER_REJECTED to
HANDOVER_FAILURE_USER_REJECTED
3. Removed the HANDOVER_FAILURE_DEST_INVALID_PERM constant since it isn't
used (methods which deal with permissions throw security exceptions).

Test: Make doc and verify documentation.
Change-Id: Id21d6b4c83d5c773ab38d78eb6b1886a1ac4dadf
Fixes: 73751004
Fixes: 73750515
Fixes: 73750817