LEDs Brightness [1/2]: Lights notifications brightness support

Implement the support of an overall brightness control for the LEDs.

The setting is deactivated by default
and will be ignored by the unimplemented phones.
Current LibLights will simply not use the new variable.

Changes includes :
  frameworks/base
  packages/apps/Settings

Change-Id: I6ff14ee0de8200f211302a868c5b43e3ec6f9f3a
Signed-off-by: AdrianDC <radian.dc@gmail.com>