Lock screen background: Update colorpicker

Additionals:

- Add colorpicker preference

- Colorpicker preference:

  - Preference (disabled), summary and color preview set to fully transparent

- Listpreference "Backround":

  - "Color fill" will enable colorpicker preferece
  - "Custom image" will disable colorpicker preferece
  - "Default Wallpaper" will disable colorpicker preferece