Add Settings management for upcoming ThemeService

This change introduces the new ThemeSettings framework, enabling management and persistence of user theme preferences. Key components include ThemeSettings (data model), ThemeSettingsManager (persistence and loading), ThemeSettingsField (individual property handling), and ThemeStyle (theme type management). The framework serializes theme data to JSON in Settings.Secure, parses JSON, and provides an API for accessing and updating theme preferences. Comprehensive tests have been added to validate core functionality, and the api is under a feature flag for future enabling.

Bug: 333694176
Test: atest FrameworksServicesTests_theme
Flag: android.server.enable_theme_service
Change-Id: I18268db0cb4ee4c0fbd753f11850eeb7230a50ac