Lockscreen Wallpaper using ImageWallpaper

Use lockscreen wallpaper image if present and keyguard
is shown.

Change-Id: I1670d0360e68afc0a50530cd07781232e809d9b9
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>
Merge "Lockscreen Wallpaper using ImageWallpaper" into jb-mr2