Set current user to USER_NULL when HAL dies

Fixes: 129527541

Test: 1) Enroll fingerprint
      2) Go to keyguard
      3) Kill fingerprint HAL
      4) Look at enumerate logs after HAL is back - no fingerprints removed
      5) Authenticate works as expected

Change-Id: I11520cd3e55ed5e7fcf0f94782a6dc4fa85735c8