According to the enrolled status to show/hide face unlock results

In current design, we only check the hardware support for face unlock to
show/hide the face unlock results in Settings Search. However the face
settings page is not launchable when the user doesn't enroll the face
unlock. It will cause user confused that face unlock results is no
response when they click them. Therefore, it's more making sense to add
enrolled status checking to index the face unlock results.

Test: manual and robotests
Fixes: 157954564
Change-Id: I5dd36e15fe48d537ee499c73cc172fc913b39554