Upgrade android_splash_screen Gradle configuration (#2779)

I noticed that the `android_splash_screen` sample was outdated and was
not runnable due to gradle issues.

This commit fixes 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* Removing the outdated `buildscript` block from the root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`.

@ericwindmill 

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).

Co-authored-by: Eric Windmill <eric@ericwindmill.com>

android_splash_screen/android/settings.gradle

@@ -1,11 +1,25 @@
-include ':app'
+pluginManagement {
+    def flutterSdkPath = {
+        def properties = new Properties()
+        file("local.properties").withInputStream { properties.load(it) }
+        def flutterSdkPath = properties.getProperty("flutter.sdk")
+        assert flutterSdkPath != null, "flutter.sdk not set in local.properties"
+        return flutterSdkPath
+    }()
 
-def localPropertiesFile = new File(rootProject.projectDir, "local.properties")
-def properties = new Properties()
+    includeBuild("$flutterSdkPath/packages/flutter_tools/gradle")
 
-assert localPropertiesFile.exists()
-localPropertiesFile.withReader("UTF-8") { reader -> properties.load(reader) }
+    repositories {
+        google()
+        mavenCentral()
+        gradlePluginPortal()
+    }
+}
 
-def flutterSdkPath = properties.getProperty("flutter.sdk")
-assert flutterSdkPath != null, "flutter.sdk not set in local.properties"
-apply from: "$flutterSdkPath/packages/flutter_tools/gradle/app_plugin_loader.gradle"
+plugins {
+    id "dev.flutter.flutter-plugin-loader" version "1.0.0"
+    id "com.android.application" version "8.6.0" apply false
+    id "org.jetbrains.kotlin.android" version "2.1.0" apply false
+}
+
+include ":app"