refactor(auth,android): migrate native implementation to Kotlin

packages/firebase_auth/firebase_auth/android/build.gradle

@@ -2,6 +2,7 @@ group 'io.flutter.plugins.firebase.auth'
 version '1.0-SNAPSHOT'
 
 apply plugin: 'com.android.library'
+apply from: file("local-config.gradle")
 
 buildscript {
     repositories {
@@ -10,13 +11,23 @@ buildscript {
     }
 }
 
-allprojects {
+rootProject.allprojects {
     repositories {
         google()
         mavenCentral()
     }
 }
 
+// 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 = findProject(':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?')
@@ -36,16 +47,21 @@ android {
       namespace 'io.flutter.plugins.firebase.auth'
     }
 
-    compileSdkVersion 34
+    compileSdkVersion project.ext.compileSdk
 
     defaultConfig {
-        minSdkVersion 23
+        minSdkVersion project.ext.minSdk
         testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
     }
 
     compileOptions {
-        sourceCompatibility JavaVersion.toVersion(17)
-        targetCompatibility JavaVersion.toVersion(17)
+        sourceCompatibility project.ext.javaVersion
+        targetCompatibility project.ext.javaVersion
+    }
+
+    sourceSets {
+        main.java.srcDirs += "src/main/kotlin"
+        test.java.srcDirs += "src/test/kotlin"
     }
 
     buildFeatures {
@@ -63,4 +79,12 @@ android {
     }
 }
 
+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")

packages/firebase_auth/firebase_auth/android/local-config.gradle

@@ -0,0 +1,7 @@
+ext {
+    compileSdk=34
+    minSdk=23
+    targetSdk=34
+    javaVersion = JavaVersion.toVersion(17)
+    androidGradlePluginVersion = '8.3.0'
+}