Start using LetterboxAppearanceCalculator in SysUI

Start using LetterboxAppearanceCalculator in
CentralSurfacesCommandQueueCallbacks, to override the status bar
appearance when apps are letterboxed.

+ Flag guarded for now
+ Also refactors all the logic that was in onSysBarAttrsChanged into its
  own class, to avoid having much logic and state in the
  CentralSurfacesCommandQueueCallbacks class

Test: CentralSurfacesCommandQueueCallbacksTest.java
Test: SystemBarAttributesListenerTest.kt
Test: Manually
Bug: 238741157
Change-Id: I31e0fa200b5da7d46a82712f5b69c4d874de72f8