Add internal listener for wallpaper color changes

This change introduces new APIs in `WallpaperManagerInternal` to allow other system services to listen for wallpaper color changes and retrieve the latest colors.

- `addOnColorsChangedListener`: Allows system components to register a callback that is invoked via a Handler whenever wallpaper colors change for any user.
- `removeOnColorsChangedListener`: Removes the listener registered with `addOnColorsChangedListener`.
- `getWallpaperColors`: Retrieves the `WallpaperColors` of either system or lock wallpapers for a given user.

`WallpaperManagerService` is updated to manage and notify these new internal listeners. Unit tests are added to verify the new functionality.

Bug: 333694176
Bug: 440236342
Test: WallpaperManagerServiceTests
Flag: EXEMPT no logical change
Change-Id: I6b0b961a10d7443dfa162898bb97c42af5d11802