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)
- Lockscreen custom image transaprency fixed (crDroid - based on kufikugel codes)

Change-Id: Idf0bbb7bf9e132a3a68d567e3863cce1b6d99742

Ps: there are some other changes made by me (based on this very same commit). This changes brings the fix in lockscreen custom image alpha. Now it's working