chore(android): support AGP 9 built-in Kotlin

Author: hiroshihorie

android/build.gradle

@@ -2,7 +2,7 @@ group = "io.livekit.plugin"
 version = "1.0-SNAPSHOT"
 
 buildscript {
-    ext.kotlin_version = "1.8.22"
+    ext.kotlin_version = "2.1.0"
     repositories {
         google()
         mavenCentral()
@@ -22,7 +22,18 @@ allprojects {
 }
 
 apply plugin: "com.android.library"
-apply plugin: "kotlin-android"
+
+// AGP 9's built-in Kotlin compiles Kotlin itself and rejects the Kotlin Gradle
+// Plugin. Apply KGP only when built-in Kotlin is NOT active: that means AGP < 9,
+// or AGP 9 with android.builtInKotlin=false (the configuration Flutter currently
+// ships by default while the ecosystem migrates).
+def agpMajor = com.android.Version.ANDROID_GRADLE_PLUGIN_VERSION.tokenize(".")[0] as int
+def builtInKotlinActive = agpMajor >= 9 &&
+    (!project.hasProperty("android.builtInKotlin") ||
+        Boolean.parseBoolean(project.property("android.builtInKotlin").toString()))
+if (!builtInKotlinActive) {
+    apply plugin: "kotlin-android"
+}
 
 android {
     if (project.android.hasProperty("namespace")) {
@@ -36,10 +47,6 @@ android {
         targetCompatibility = JavaVersion.VERSION_1_8
     }
 
-    kotlinOptions {
-        jvmTarget = JavaVersion.VERSION_1_8
-    }
-
     sourceSets {
         main.java.srcDirs += "src/main/kotlin"
         test.java.srcDirs += "src/test/kotlin"
@@ -68,3 +75,19 @@ android {
         }
     }
 }
+
+// Configure the Kotlin JVM target. The compilerOptions DSL requires KGP 1.9+ or
+// AGP 9 built-in Kotlin; older Flutter app templates ship KGP 1.8.x, which only
+// supports the legacy kotlinOptions DSL.
+def kotlinExt = project.extensions.findByName("kotlin")
+if (kotlinExt?.hasProperty("compilerOptions")) {
+    kotlin {
+        compilerOptions {
+            jvmTarget = org.jetbrains.kotlin.gradle.dsl.JvmTarget.JVM_1_8
+        }
+    }
+} else {
+    android.kotlinOptions {
+        jvmTarget = JavaVersion.VERSION_1_8.toString()
+    }
+}