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.
Fixes: 73751004
Fixes: 73750515
Fixes: 73750817
Merged-In: I7860fcd813f25adaaccf632f2c61dd4138a0a889
Change-Id: I7860fcd813f25adaaccf632f2c61dd4138a0a889
(cherry picked from commit c7a86b14)