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: I5bc337698bef682cdf67940906d24842e1dffc28