ButtonSettings: Rework buttons/keyboard backlight control enablement

Let's end once and for all with the nonsense of using floats
config_{button,keyboard}BrightnessSettingDefaultFloat to enable
button and keyboard brightness control, respectively.
Instead, just move over to the recently introduced integers
config_deviceSupports{Button,Keyboard}BrightnessControl.

This avoids people with devices with hardware keys having to
overlay config_{button,keyboard}BrightnessSettingDefaultFloat to
a value of 0.0 just to disable the unsupported features.

Change-Id: I0dfd0b5fa51c51398face208f17df70419ef00ca