Upgrade macOS, iOS and Android runners (#1425)

Just removed the old `ios/`, `macos/` and `android/` folders and recreated them using `flutter create .`, then massaged the runners back into working order.

game_template/android/app/build.gradle

@@ -27,6 +27,7 @@ apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
 
 android {
     compileSdkVersion flutter.compileSdkVersion
+    ndkVersion flutter.ndkVersion
 
     compileOptions {
         sourceCompatibility JavaVersion.VERSION_1_8
@@ -44,6 +45,8 @@ android {
     defaultConfig {
         // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
         applicationId "com.example.game_template"
+        // You can update the following values to match your application needs.
+        // For more information, see: https://docs.flutter.dev/deployment/android#reviewing-the-build-configuration.
         minSdkVersion 21
         targetSdkVersion flutter.targetSdkVersion
         versionCode flutterVersionCode.toInteger()