Retrieve authenticatorId when already targetUserId

Previous change ag/13413165 stopped the framework from retrieving
authenticatorId when the current user is already the target user.
However, this happens immediately after enrollment, since enroll
and getAuthenticatorId are separate operations. So, add a way to
retrieve authenticatorId only when necessary.

Fixes: 179737483
Bug: 178018968
Test: manual
Change-Id: I71673fc672c60733616634a2fdf3153a3b2d02e1