Fix build.

 * A wallpaper service is responsible for showing a live wallpaper behind

 * applications that would like to sit on top of it.  This service object

 * itself does very little -- its only purpose is to generate instances of

 * {@link WallpaperSerice.Engine} as needed.  Implementing a wallpaper thus

 * {@link Engine} as needed.  Implementing a wallpaper thus

 * involves subclassing from this, subclassing an Engine implementation,

 * and implementing {@link #onCreateEngine()} to return a new instance of

 * your engine.