Configurable letterbox background.

Allow three types of the letterbox wallpaper:
- Solid background using color specified in R.color.config_letterboxBackgroundColorSolid color
- Color specified in R.attr.colorBackground for the letterboxed application
- Color specified in R.attr.colorBackgroundFloating for the letterboxed application

Also, adding ADB commands for each config value:
- adb shell cmd window set-letterbox-background-color [reset|color]
- adb shell cmd window get-letterbox-background-color
- adb shell cmd window set-letterbox-background-type [reset|solid_color|app_color_background|app_color_background_floating]
- adb shell cmd window get-letterbox-background-type

Test: manual with ADB commands, atest LetterboxTest
Fix: 179088234
Change-Id: I23cf0b52c9f95d1f92255731bc7be165a575c7cd