Framework : Lockscreen background customization (1/2)

Allow user to customize lockscreen background (port from cm7)

Part 2 : http://review.cyanogenmod.com/#/c/16914/
Part 3 : Minor edits
Part 4 : Fix crash on tablets. Added nullcheck to ViewGroup Layout
Part 5 : Add missing id to layout
Part 6 : Reordering/renaming of setBackground

Change-Id: I3f904486b8c7bcfcb63f54937810f6ee9c1721ef