android: Migrate Flutter project to new Gradle plugin syntax

Migrate Flutter project to the new Gradle plugin syntax recommended in Flutter 3.16.

* Updated settings.gradle to use the pluginManagement and plugins blocks instead of the legacy apply script method.
* Removed the buildscript block from android/build.gradle.
* Removed the legacy apply plugin statements from android/app/build.gradle.
* Removed the unnecessary dependencies block for Kotlin in android/app/build.gradle.

Benefits:

* Improves readability and maintainability of Gradle build files.
* Makes it easier for the Flutter team to develop and release new features.
* Prepares the project for future changes in Flutter Gradle plugin support.

References:
* Flutter Gradle Plugin Apply: https://flutter.dev/go/flutter-gradle-plugin-apply

src/ui/flutter_app/android/app/build.gradle

@@ -1,3 +1,9 @@
+plugins {
+    id "com.android.application"
+    id "kotlin-android"
+    id "dev.flutter.flutter-gradle-plugin"
+}
+
 def localProperties = new Properties()
 def localPropertiesFile = rootProject.file('local.properties')
 if (localPropertiesFile.exists()) {
@@ -6,11 +12,6 @@ if (localPropertiesFile.exists()) {
     }
 }
 
-def flutterRoot = localProperties.getProperty('flutter.sdk')
-if (flutterRoot == null) {
-    throw new GradleException("Flutter SDK not found. Define location with flutter.sdk in the local.properties file.")
-}
-
 def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
 if (flutterVersionCode == null) {
     flutterVersionCode = '1'
@@ -21,9 +22,6 @@ if (flutterVersionName == null) {
     flutterVersionName = '1.0'
 }
 
-apply plugin: 'com.android.application'
-apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
-
 android {
     compileSdkVersion 34
     namespace "com.calcitem.sanmill"

src/ui/flutter_app/android/app/build.gradle_fdroid

@@ -1,3 +1,9 @@
+plugins {
+    id "com.android.application"
+    id "kotlin-android"
+    id "dev.flutter.flutter-gradle-plugin"
+}
+
 def localProperties = new Properties()
 def localPropertiesFile = rootProject.file('local.properties')
 if (localPropertiesFile.exists()) {
@@ -6,11 +12,6 @@ if (localPropertiesFile.exists()) {
     }
 }
 
-def flutterRoot = localProperties.getProperty('flutter.sdk')
-if (flutterRoot == null) {
-    throw new GradleException("Flutter SDK not found. Define location with flutter.sdk in the local.properties file.")
-}
-
 def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
 if (flutterVersionCode == null) {
     flutterVersionCode = '1'
@@ -21,9 +22,6 @@ if (flutterVersionName == null) {
     flutterVersionName = '1.0'
 }
 
-apply plugin: 'com.android.application'
-apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
-
 android {
     compileSdkVersion 34
     namespace "com.calcitem.sanmill"

src/ui/flutter_app/android/app/build.gradle_github

@@ -1,3 +1,9 @@
+plugins {
+    id "com.android.application"
+    id "kotlin-android"
+    id "dev.flutter.flutter-gradle-plugin"
+}
+
 def localProperties = new Properties()
 def localPropertiesFile = rootProject.file('local.properties')
 if (localPropertiesFile.exists()) {
@@ -6,11 +12,6 @@ if (localPropertiesFile.exists()) {
     }
 }
 
-def flutterRoot = localProperties.getProperty('flutter.sdk')
-if (flutterRoot == null) {
-    throw new GradleException("Flutter SDK not found. Define location with flutter.sdk in the local.properties file.")
-}
-
 def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
 if (flutterVersionCode == null) {
     flutterVersionCode = '1'
@@ -21,9 +22,6 @@ if (flutterVersionName == null) {
     flutterVersionName = '1.0'
 }
 
-apply plugin: 'com.android.application'
-apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
-
 def keystoreProperties = new Properties()
 def keystorePropertiesFile = rootProject.file('key.properties')
 if (keystorePropertiesFile.exists()) {

src/ui/flutter_app/android/build.gradle

@@ -1,20 +1,3 @@
-buildscript {
-    ext.kotlin_version = '1.8.22'
-    repositories {
-	    // Gradle 4.1 and higher include support for Google's Maven repo using
-        // the google() method. And you need to include this repo to download
-        // Android Gradle plugin 3.0.0 or higher.
-        google()
-        mavenCentral()
-    }
-
-    dependencies {
-        classpath 'com.android.tools.build:gradle:8.1.0'
-		classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
-        classpath "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
-    }
-}
-
 allprojects {
     repositories {
         google()

src/ui/flutter_app/android/settings.gradle

@@ -1,15 +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 flutterProjectRoot = rootProject.projectDir.parentFile.toPath()
+    includeBuild("$flutterSdkPath/packages/flutter_tools/gradle")
 
-def plugins = new Properties()
-def pluginsFile = new File(flutterProjectRoot.toFile(), '.flutter-plugins')
-if (pluginsFile.exists()) {
-    pluginsFile.withReader('UTF-8') { reader -> plugins.load(reader) }
+    repositories {
+        google()
+        mavenCentral()
+        gradlePluginPortal()
+    }
 }
 
-plugins.each { name, path ->
-    def pluginDirectory = flutterProjectRoot.resolve(path).resolve('android').toFile()
-    include ":$name"
-    project(":$name").projectDir = pluginDirectory
+plugins {
+    id "dev.flutter.flutter-plugin-loader" version "1.0.0"
+    id "com.android.application" version "8.1.0" apply false // Replace with your AGP version
+    id "org.jetbrains.kotlin.android" version "1.8.22" apply false // Replace with your Kotlin version
 }
+
+include ":app"