Freeze live wallpaper for letterbox background.

With this change, WallpaperController sends "freeze" / "unfreeze" commands when the app window is letterboxed / not letterboxed indicating that:
- "freeze": a screenshot of the wallpaper is taken by WallpaperServices and put on top of the main surface while the wallpaper host is notified through onVisibilityChanged(false) callback that it's no longer visible.
- "unfreeze": a screenshot of the wallpaper is removed and the wallpaper host is notified through onVisibilityChanged(true) callback if the wallpaper is visible again (e.g., if transitioned from an app to the Home screen).

Logic isn't applied to static wallpapers.

When wallpaper size changes while being frozen, the screenshot is scaled and cropped to adjust to the new size.

Fix: 177884751
Test: Manual, atest WallpaperServiceTest, WallpaperServiceTests
Change-Id: If0300478cdccc07d226117e01ce3d43aa0b432a0