[5/n] Create LetterboxConfiguration component

Creates the LetterboxConfiguration for the properties related to the
background colors.

Flag: com.android.window.flags.app_compat_refactoring
Bug: 370940063
Test: atest WMShellUnitTests:LetterboxConfigurationTest

Change-Id: I522904f4f93cf9388e3959f3f36a1a00ecf84c4d