Introduce state management dependencies for ThemeService

Adds the core data structures and logic for managing user-specific theme states.

- `ThemeState`: An immutable record to hold a user's theme settings.
- `ThemeStatePair`: Manages current and pending `ThemeState`, containing the logic to determine when to update overlays and handle deferrals.
- `ThemeInfo`: A parcelable used for IPC to request theme changes.

This change also includes comprehensive unit tests for the new components and a `FakeScheduledExecutorService` for testing.

Bug: 333694176
Test: atest FrameworksServicesTests_theme
Flag: android.server.enable_theme_service
EXPECTED_PROCESS_CRASH=system_server

Change-Id: I692a408a6e5479853182e2dc3f307518306d0f7b