Migrate to declarative Gradle plugin application

Update example app to use the new declarative plugins block instead of
imperative apply methods, as required by Gradle 8.x and modern Flutter.

Changes:
- Migrate settings.gradle to use pluginManagement and plugins blocks
- Update app/build.gradle to use plugins block
- Fixes "app_plugin_loader Gradle plugin imperatively" error

This is required for Gradle 8.1+ compatibility.

Author: claude

example/android/app/build.gradle

@@ -21,9 +21,11 @@ if (flutterVersionName == null) {
     flutterVersionName = '1.0'
 }
 
-apply plugin: 'com.android.application'
-apply plugin: 'kotlin-android'
-apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
+plugins {
+    id "com.android.application"
+    id "org.jetbrains.kotlin.android"
+    id "dev.flutter.flutter-gradle-plugin"
+}
 
 android {
     compileSdkVersion 34

example/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 "8.1.0" apply false
+    id "org.jetbrains.kotlin.android" version "1.7.10" apply false
+}
+
+include ":app"