[1/4] framework: custom brightness support

Adds separate handling of button brightness for devices
that supports it. Enabled with config_button_brightness_support
else the behaviour will be the same as before

Also if enabled but no custom values are set will fallback
to the previous behaviour

This also needs support of setting button brightness separately
in liblights code of the device.

Options:
-brightness spline for buttons in automatic mode
-custom brightness value for buttons in manual mode
-options to competely disable buttons in both modes
-option to always fallback to same value as screen brightness

This also brings in:
-support for custom responsivness value for automatic mode
-custom twilight support enablement

PS2:
-added API for settings

PS4:
-whitespace cleanup

Change-Id: I37535de80bd30936319f3e2535e991f7c6c195c1
Merge "[1/4] framework: custom brightness support" into android-4.3