sdk: Move to new button and keyboard brightness configs

For as strange as it may be, so far the visibility of the features depended
on the default value of config_{button,keyboard}BrightnessSettingDefaultFloat.
This was nonsense, because it would force everyone to overlay that default
with a value of 0.0, just to make the unsupported feature go away.

Change-Id: If605238bf30642db057b06b03224d9a8b4451f12