Support dynamic material color for letterbox background.

- Use SystemUI context to access overlays with material colors extracted
from wallpaper.
- Allow to set color with a resource name via an ADB command.
- Set background color to system_neutral2_500

Bug: 197361088
Test: atest WmTest:SizeCompatTests and manual with adb shell cmd window set-letterbox-style --backgroundColorResource @android:color/resource_name
Change-Id: I6aa593b6697fc9846542739427eb09168cdb038b