WIP Settings: Lockscreen custom wallpaper (2/2)

  - Color Fill
  - Custom Wallpaper
  - Default background
  - Alpha slider in color fill

patchset 2: rebased...move settings strings to Settings.System
            also set right value for colorpicker preview

patchset 3: fix image handling based on idea borrowed from AOKP, also remove lockscreen alpha settings

patchset 4: remove unused file and import

Change-Id: I7d02feed71e57e6af07e19f50e18a09d03f1dfcc