CM11 Themes: Allow defining a custom default theme [2/3]

Change-Id: I94521a09339fb760b7a56b9a3ba3d2c56e2d3ac5
Properly handle updating holo theme.

Change-Id: I157e597ee53894b1237c05dd2000b1da957b1968
Automatic translation import

Change-Id: Idc73415b665b719cba153929000cc960993241d9
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