Settings: Hide Flashlight

* Class comment states: "Headless activity that toggles
  flashlight state when launched."
* Comment in constructor states:
  "The sole purpose of this activity is to provide a place
   to index flashlight into Settings search."
* Since it does only the second thing successfully and we
  do neither know why we need a useless search result nor
  does it server any purpose, hide it from search results
* We could remove the whole class, but since it's from AOSP
  I decided to keep the change footprint minimal

Test: Type "Flashlight" into settings search - result does not
appear anymore
Fixes: https://gitlab.com/LineageOS/issues/android/-/issues/6397

Change-Id: I53541c700e24efd33d386c38a4ea38fb1b8b81d6