Limit number of fingerprint templates that can be enrolled per device user

This change places an upper limit on the number of fingerprint templates that
can be enrolled per account.  This is done primarily for performance reasons, but
may also be imposed by hardware and Trusted Execution Environment (TEE) reasons.

Fixes bug 20731847

Change-Id: I3975290fa8eb2106467493aab6102015697e012c

 On branch max_fingerprint