Fix plugins block ordering in app build.gradle

Move plugins block to the top of build.gradle file as required by
Gradle 8.x. The plugins block must be the first statement, before
any variable definitions or other code.

Fixes: "only buildscript {}, pluginManagement {} and other plugins {}
script blocks are allowed before plugins {} blocks"

Author: claude

example/android/app/build.gradle

@@ -1,3 +1,9 @@
+plugins {
+    id "com.android.application"
+    id "org.jetbrains.kotlin.android"
+    id "dev.flutter.flutter-gradle-plugin"
+}
+
 def localProperties = new Properties()
 def localPropertiesFile = rootProject.file('local.properties')
 if (localPropertiesFile.exists()) {
@@ -21,12 +27,6 @@ if (flutterVersionName == null) {
     flutterVersionName = '1.0'
 }
 
-plugins {
-    id "com.android.application"
-    id "org.jetbrains.kotlin.android"
-    id "dev.flutter.flutter-gradle-plugin"
-}
-
 android {
     compileSdkVersion 34
     ndkVersion flutter.ndkVersion