packages/flutter_tools/templates/module/android/host_app_common/app.tmpl/src/main/res/values/styles.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <style name="LaunchTheme" parent="@android:style/Theme.Black.NoTitleBar">
        <!-- Show a splash screen on the activity. Automatically removed when
             the Flutter engine draws its first frame -->
        <item name="android:windowBackground">@drawable/launch_background</item>
    </style>
</resources>