[CS] 4/ Small updates to LetterboxAppearanceCalculator APIs.

The next CL will move the status bar mode & appearance calculations
which require some updates to the LetterboxAppearanceCalculator APIs.
This CL makes those API changes so that the next CL is slightly cleaner.

Changes:
 - Use List instead of Array. (Kotlin data classes require List instead
   of Array and the upcoming repo will use data classes)
 - Directly fetch the icon colors instead of getting them from
   LightBarController. (Removing this LightBarController dependency
   means that the next CL won't have a circular dependency)

Bug: 300519002
Test: smoke test of letterboxing effect on status bar icons
Test: dump LetterboxAppearanceCalculator
Test: atest LetterboxAppearanceCalculatorTest
Change-Id: Ifafceabd19de86961693136e26df5314728340ca