Bring back the custom wallpaper picker [1/5]

This is the WallpaperPicker found in Trebuchet, with the following
key features:

  - Set launcher wallpaper as well as lock screen wallpaper
  - Direct support for "no wallpaper"
  - Loads wallpapers from themes ;)
  - Supports partner wallpapers

Change-Id: I6992a5017694bae5023cddf34db576da8795021e
TICKET: OSS-62