refactor(app_installations): migrate Android plugin to Kotlin

packages/firebase_app_installations/firebase_app_installations/android/build.gradle

@@ -5,10 +5,14 @@ apply plugin: 'com.android.library'
 apply from: file("local-config.gradle")
 
 buildscript {
+    ext.kotlin_version = "2.0.0"
     repositories {
         google()
         mavenCentral()
     }
+    dependencies {
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
+    }
 }
 
 rootProject.allprojects {
@@ -18,6 +22,16 @@ rootProject.allprojects {
     }
 }
 
+// AGP 9+ has built-in Kotlin support unless Flutter opts out via android.builtInKotlin=false.
+def agpMajor = com.android.Version.ANDROID_GRADLE_PLUGIN_VERSION.tokenize('.')[0] as int
+def builtInKotlin = providers.gradleProperty("android.builtInKotlin")
+        .map { it.toBoolean() }
+        .orElse(agpMajor >= 9)
+        .get()
+if (agpMajor < 9 || !builtInKotlin) {
+    apply plugin: 'kotlin-android'
+}
+
 def firebaseCoreProject = rootProject.subprojects.find { it.name == "firebase_core" }
 if (firebaseCoreProject == null) {
   throw new GradleException('Could not find the firebase_core FlutterFire plugin, have you added it as a dependency in your pubspec?')
@@ -49,6 +63,11 @@ android {
       targetCompatibility project.ext.javaVersion
     }
 
+    sourceSets {
+      main.java.srcDirs += "src/main/kotlin"
+      test.java.srcDirs += "src/test/kotlin"
+    }
+
     buildFeatures {
         buildConfig true
     }
@@ -64,4 +83,12 @@ dependencies {
   implementation 'androidx.annotation:annotation:1.7.0'
 }
 
+plugins.withId("org.jetbrains.kotlin.android") {
+  kotlin {
+    compilerOptions {
+      jvmTarget = org.jetbrains.kotlin.gradle.dsl.JvmTarget.fromTarget(project.ext.javaVersion.toString())
+    }
+  }
+}
+
 apply from: file("./user-agent.gradle")