[6/n] Implement LetterboxCommandHandler

Implements the ShellCommandActionHandler to handle the
backgroundColor and backgroundColorResource options.

The command to run is
adb shell dumpsys activity service SystemUIService WMShell
   letterbox <command>

Flag: com.android.window.flags.app_compat_refactoring
Bug: 370940685
Test: m and manual

Change-Id: I07e8efcca785889d6270a475d8b5ca6b6e7aed16