FingerprintService: fix fingerprints disappearing

Whenever getEnrolledFingerprints() would be called the service would try
and remove any native fingerprints no longer in settings. This could
potentially remove fingerprints for another user.

Restrict fingerprints to primary users for now.

Ref: CYNGNOS-1251

Change-Id: I266af32b16026298e78ece8637f5922795bf3196
Signed-off-by: Roman Birg <roman@cyngn.com>