Frameworks: simplify custom lockscreen background code

- remove observer for transparency and handle it better
- add multisuser handles
- smaller fixes and code cleanup
- simpfliy custom background handling and change to our needs (thanks Danish)

Change-Id: Idf0bbb7bf9e132a3a68d567e3863cce1b6d99742

Conflicts:
	policy/src/com/android/internal/policy/impl/keyguard/KeyguardHostView.java
	policy/src/com/android/internal/policy/impl/keyguard/KeyguardViewManager.java
	services/java/com/android/server/wm/WindowAnimator.java