[1/2] 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

Change-Id: I79cc8cf3a9b488b857929f327cb67a770ec81916

Conflicts:
	services/java/com/android/server/power/DisplayPowerController.java

Fix?