Fix bug where fingerprint for wrong userId was attempted to be removed.

- cleaned up private API to ensure userId is distinct from groupId.
- fixed bug where we were sending the wrong userId when attempting to
- fix warning about wrong fingerId when receiving final id of 0.

Fixes bug 28268635

Change-Id: Ic8abfbf6fbf173db2d57a76ac2e38b2a71ffd19e