Reduce calling getLowResBitmap() times

- Optimize quick switch preference storing without background thread
  and remove calling getLowResBitmap().
- Use a new Future in WallpaperInfo to get wallpaper color and
  placeholder color.

Bug: 219399108
Test: manual
Change-Id: I63bb53d974bd79b7a8bb32ad1400147de9eacb05
7 files changed