Standardize preview images.

Themes can contain a preview image for wallpaper, lockscreen, and
styles.  These can be either .png or .jpg and use the following
naming conventions:
  wallpaper  -> assets/images/wallpaper_preview.[png|jpg]
  lockscreen -> assets/images/lockscreen_preview.[png|jpg]
  styles     -> assets/images/styles_preview.[png|jpg]

Change-Id: Ibe441abf941e8b995c94eb43f6c6f237758d914b