LEDs Brightness [2/2]: 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: I1c0de01b1c6a2a2cf1432028a2e69f90b2373b2c
Signed-off-by: AdrianDC <radian.dc@gmail.com>