Fingerprint: get auth id for non-current user.

Previously, getAuthenticatorId() simply returns the authenticator id
corresponding to the currently active user in FingerprintService.
However, this can cause bugs when, for example, KeyStore calls the
method before storing a fingerprint-bound key for a non-current user. In
such cases, the authenticator id of the calling user is desired, which
is not necessarily the same as the "current user" in FingerprintService.
This CL ensures the FingerprintService always returns the authenticator
id of the calling user.

Bug: 33459191
Test: manual
Change-Id: Ia9d6b869d16bd37f45358ba839cd12901ebc1076
Merged-In: I35c5a3a7082cffb8941eeaa219c8e20948ad41a9