Wallpaper background for letterbox.

Changes:
- Allows to request 'wallpaper' background for letterbox and to specify blur radius and alpha of dark scrim that will dim the wallpaper in config overlays or via ADB commands.
- To propagate "showing wallpaper for letterbox" property to the rest of system independently from FLAG_SHOW_WALLPAPER
- Checks mShowWallpaperForLetterboxBackground in all places where FLAG_SHOW_WALLPAPER is checked.
- Doesn't show wallpaper background when letterboxed only for display cutout.

Fix: 170215478
Fix: 181679062
Test: atest LetterboxTest and manual with ADB commands
- adb shell cmd window set-letterbox-background-type wallpaper
- adb shell cmd window set-letterbox-background-wallpaper-blur-radius <radius>
- adb shell cmd window set-letterbox-background-wallpaper-dark-scrim-alpha <alpha>

Change-Id: Ic5902b085c0a9d250107c3b5ca2d329a9f47c71b