[2/2] Improve flash availability checking and caching.
* On tablets without flash invoking a camera instance for checking flash availability is not needed,
  it will delay the result (false) and then turn on the white screen without taking advantage of the invoked
  camera to toggle the flash, differently from devices that have it. Add a boolean to override for devices with
  flash in their device trees.
  A similar change is done in frameworks as well, different but related to avoid delays.

Signed-off-by: Carlo Savignano <stevewatersy@gmail.com>