Dark theme toggle (2/2)

Implement using clean architecture and preview & apply pattern on the
picker side. Create an activity-scoped DarkModeLifecycleUtil to update
the singleton-scoped DarkModeRepositoryImpl during lifecycle changes.

Does not include changes to preview dark theme colors on the
picker UI or on Launcher.

Flag: com.android.systemui.shared.new_customization_picker_ui
Test: manually verified with battery saver mode on and off
Test: DarkModeViewModelTest
Bug: 371985937
Change-Id: I7b9c8a5f3e550c8c15ed6e1d5728e8f897ee2e8e