Add Null check FingerprintSettings#updateAddPreference()

When fingerprint finish enrollment, user click Done
The flow would be:
-> wm_finish_activity
-> relaunch_resume_activity
-> wm_on_destroy_called
-> wm_on_create_called
-> wm_on_start_called
-> wm_on_activity_result_called
   In U, we need to add NPE check when updateAddPreference()
   earlier then addFingerprintItemPreferences()
-> wm_on_resume_called

Bug: 275519315
Test: manual enroll SFPS, in FingerprintEnrollEnrolling,
      rotate device, then complete the enrollment
      verify the flow never crash and user can see enrolled
      fingerprint in FingerprintSettings

Change-Id: Ia262741aeceeb36eefd159ad2ca1c9fa0e061d54