Settings: Custom lock screen background [2/2]

Patch Set 1:    * port the custom lock screen feature from
                  ics (http://review.cyanogenmod.org/#/c/16914)

Patch Set 2:    * rename some strings
                * use constants instead of numbers
                * fix some code styling issues

Patch Set 3:    * cleanup

Patch Set 4:    * rebased

Change-Id: I35e5c70fff423770a03b5bb39d439ebea82dd56a