Frameworks: simplify custom lockscreen background code + Frameworks: use a layer drawable for the keyguard

simplify lockscreen part:
- 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)
- Lockscreen custom image transaprency fixed (crDroid - based on kufikugel codes)

Change-Id: Idf0bbb7bf9e132a3a68d567e3863cce1b6d99742

use a layer drawable part:
in preperation to the new navbar and statusbar transparency
managment which will come it is more consistent on custom image
and color fill on lockcreen to use a layerdrawable to add the
shadow on top of the image to keep on transparency of the background
the shadow like it is suggested by google

Change-Id: I22df7652b8be42cab1c9d1f0a13797da40a75dd9