WIP Frameworks: Lockscreen custom wallpaper (1/2)

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

patchset 2: rebased...move settings from Settings.Secure to Settings.System
            also don't set album art when transport is active

patchset 3: != mCustomBackground

patchset 4: fix image handling

patchset 5: remove unused import

Change-Id: I47ac785cd650bafe218cd9417b890c31e3a234cf