This commit modernizes the Gradle setup for the `android_splash_screen` sample.
Key changes include:
* Upgrading Gradle wrapper from version 6.7 to 8.7.
* Migrating to the plugins block in `settings.gradle` and `app/build.gradle` for applying Android, Kotlin, and Flutter Gradle plugins.
* Removing the outdated `buildscript` block from the root `build.gradle`.
* Updating `compileSdkVersion` and `targetSdkVersion` to 34.
* Setting `minSdkVersion` to `flutter.minSdkVersion`.
* Adding the `namespace` property as required by newer Android Gradle Plugin versions.
* Refactoring Kotlin constant declarations in `MainActivity.kt`.
Adds a Flutter app designed for devices running at least Android 12 that exemplifies how to implement an animated splash screen with the SplashScreen API.
NiazSagor
Camille Simon