Frameworks: use a layer drawable for the keyguard

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