Keyguard: Add customizable lockscreen background

 * Loosely based on: http://goo.gl/hmf76
 * Can set a fill color, custom image and completely transparent with
   desktop showing.

Change-Id: I8567d3f46d2ce32feecad55da7896592fb3c423c

Conflicts:
	core/java/android/provider/Settings.java
	packages/Keyguard/src/com/android/keyguard/KeyguardViewManager.java
	services/java/com/android/server/wm/WindowAnimator.java