frameworks: add keyguard wallpaper functionality to WallpaperManager

Adapted for cm-12.

Change-Id: I0eb1b5578b705c2ade4c0772fa86e09478b9f73e
Signed-off-by: Roman Birg <roman@cyngn.com>
(cherry picked from commit 1f77af99a4911aec2c162233f95aa868e1218fbe)