Migrating from Manifest / Activity defined custom splash screens

[prestosole]

From https://docs.flutter.dev/development/ui/advanced/splash-screen :

Previously, Android Flutter apps would either set io.flutter.embedding.android.SplashScreenDrawable in their application manifest, or implement provideSplashScreen within their Flutter Activity. This would be shown momentarily in between the time after the Android launch screen is shown and when Flutter has drawn the first frame. This is no longer needed and is deprecated – in Flutter 2.5 and later, Flutter automatically keeps the Android launch screen displayed until Flutter has drawn the first frame. Developers should instead remove usage of these APIs.