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