Update Android app project template to apply Kotlin Gradle plugin declaratively (#139006)

This PR updates the app templates generated by `flutter create` to use declarative `plugins {}` syntax for applying the Kotlin Gradle Plugin.

I realized this is missing while writing [#9857.](https://github.com/flutter/website/pull/9857)

packages/flutter_tools/templates/app_shared/android-java.tmpl/build.gradle.tmpl

-buildscript {
-    ext.kotlin_version = '{{kotlinVersion}}'
-    repositories {
-        google()
-        mavenCentral()
-    }
-
-    dependencies {
-        // AGP version is set in settings.gradle.
-        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
-    }
-}
-
 allprojects {
     repositories {
         google()

packages/flutter_tools/templates/app_shared/android-kotlin.tmpl/app/src/main/kotlin/androidIdentifier/MainActivity.kt.tmpl

@@ -2,5 +2,4 @@ package {{androidIdentifier}}
 
 import io.flutter.embedding.android.FlutterActivity
 
-class MainActivity: FlutterActivity() {
-}
+class MainActivity: FlutterActivity()

packages/flutter_tools/templates/app_shared/android-kotlin.tmpl/build.gradle.tmpl

-buildscript {
-    ext.kotlin_version = '{{kotlinVersion}}'
-    repositories {
-        google()
-        mavenCentral()
-    }
-
-    dependencies {
-        // AGP version is set in settings.gradle.
-        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
-    }
-}
-
 allprojects {
     repositories {
         google()

packages/flutter_tools/templates/app_shared/android.tmpl/settings.gradle.tmpl

@@ -20,6 +20,7 @@ pluginManagement {
 plugins {
     id "dev.flutter.flutter-plugin-loader" version "1.0.0"
     id "com.android.application" version "{{agpVersion}}" apply false
+    id "org.jetbrains.kotlin.android" version "{{kotlinVersion}}" apply false
 }
 
 include ":app"