Improve button/keyboard backlight code.

- Improve UX
- Deduplicate code

Conflicts:
        res/layout/button_backlight.xml
        res/layout/keyboard_backlight.xml
        res/values-da/cm_strings.xml
        res/values-el/cm_strings.xml
        res/values-es/cm_strings.xml
        res/values-hu/cm_strings.xml
        res/values-it/cm_strings.xml
        res/values-pt/cm_strings.xml
        res/values-ru/cm_strings.xml
        res/values/cm_strings.xml
        res/xml/button_settings.xml
        src/com/android/settings/cyanogenmod/ButtonBacklightBrightness.java
        src/com/android/settings/cyanogenmod/ButtonSettings.java
        src/com/android/settings/cyanogenmod/KeyboardBacklightBrightness.java

Change-Id: Ie0218de8b83fe49e3fe1d91a64c2522af03ba06d