Added settings activity sans interaction logic

 * Registered activity as Muzei settings activity for art source.
 * Added themes/styles used in settings activity.
 * Added layout for settings activity.
   * Layout uses tools:textColor to style radio buttons in Android Studio
     preview to work around inability of previews to correctly use the color
     state list.
Implemented interaction logic for settings activity

 * Integrated UI with preferences storage
 * Integrated art source with preferences storage
 * Added up affordance to settings UI: currently, this simply finishes the
   activity, as the only valid entry point is from Muzei itself.
 * Some code cleanup.