[StatusBar][Letterbox] Retrieve letterbox background info from WM

Before this change, we were assuming that the background was always
black.
Now we retrieve the actual values from WindowManager.

This is used to determine the color of the status bar icons that are
positioned on the background.

Fixes: 238607453
Test: Manually
Test: LetterboxBackgroundProviderTest.kt
Test: LetterboxAppearanceCalculatorTest.kt
Change-Id: I9dd9c95b35c5ca7ce20ee670e073cbad578ab5d9