FingerprintEnrollFindSensor: Don't overlay front for side sensors

* Both the base image and the base image + overlay are equally nonsense
  for devices with a side fingerprint scanner, but for those that overlay
  a custom fingerprint enrollment graphic, this image overlay wreaks havoc
* Only set this overlay to be visible if the sensor is on the front, to
  allow devices with side sensors to only overlay the base image

Change-Id: I7cbcea4830d4526cf9f7d139e7f84f21117fa9c4