Fix button brightness preview.

Change-Id: I2b0933062d443855114088e1bb4c09c68a83a0e5
Save value to shared prefs in HW value scale.

Otherwise we can run into precision issues if the HW e.g. uses a 0..255
scale.

Change-Id: I2df81db728faa64ee7b09cc3101122ff5c95b7a3

Conflicts:
	res/xml/more_device_settings.xml