LockSettingsService: clear strong auth when adding primary credential.

This will allow newly-created secondary users to use biometrics
immediately after setting up LSKF + Biometrics flow, and avoid future
gaps in clearing strong auth state on any new user auth fixtures that
configure a new LSKF.

Bug: 320817991
Test: atest com.android.server.locksettings with flag on/off
Flag: android.security.clear_strong_auth_on_add_primary_credential
Change-Id: I9fb08fd34993dab6ff5f9fa23880c6860e574514