Added ColorExtractionService and ExtractedColors.

- Launcher has an instance of ExtractedColors, which is loaded from
  LauncherProvider in onCreate() and whenever the wallpaper changes.
  - When the wallpaper changes, the ColorExtractionService is started
    in the :wallpaper-chooser process.
  - ColorExtractionService builds an ExtractedColors instance and saves
    it as a String in LauncherProvider.
  - When the results are saved, Launcher gets a callback through
    LauncherProviderChangeListener and reloads the ExtractedColors.
- Whenever Launcher loads Extractecolors, it also re-colors items
  (currently a no-op).

Change-Id: I319e2cfe0a86abcbc6bb39ef6b9fbbcad54ad743