WallpaperColor's ColorHint API go for @SystemApi

Making color hint's relative api @SystemApi.
For pre-installed live wallpaper apps, change to @SystemApi so:

1. apps can be build with gradle build tools.
   (e.g.: PixelLiveWallpaper for app bundle feature).
2. apps can calculate and pass color hint information to interested system components.
   (Such as passing dark text support to SystemUi)

Fixes: 111395593

Test: make system sdk and check if the wallpaper apps can compile with android_system.jar.
Test: check whether the wallpaper apps can pass correct color hint after build (dark text).

Change-Id: I2ecf6ff77ff7a823949c6e3253a077e6eba9683a