fix: apply kotlin-android when builtInKotlin=false on AGP 9+

[proninyaroslav]

Problem

Build fails with Could not find method kotlin() (build.gradle:58) when using:

• AGP 9.x
• Flutter ≥ 3.44

Flutter 3.44 introduced a migration phase where it automatically sets android.builtInKotlin=false in gradle.properties (via the Flutter migrator tool). This flag disables AGP 9's built-in Kotlin support, so the project still relies on the explicit kotlin-android plugin.

However, the plugin's condition if (agpMajor < 9) skips applying kotlin-android for all AGP 9+ projects — including those where builtInKotlin=false is set. As a result, nobody compiles the plugin's Kotlin sources and FlutterLibphonenumberPlugin class cannot be found at link time.

Fix

1. Condition: also apply kotlin-android when android.builtInKotlin is explicitly 'false', even on AGP 9+:

   def builtInKotlin = project.findProperty('android.builtInKotlin')
   if (agpMajor < 9 || builtInKotlin == 'false') {
       apply plugin: 'kotlin-android'
   }
   

2. Guard: wrap the kotlin {} compiler-options block in plugins.withId() so it only executes when the Kotlin plugin is actually present (safe for both the old path and the built-in Kotlin path):

   plugins.withId("org.jetbrains.kotlin.android") {
       kotlin {
           compilerOptions {
               jvmTarget = org.jetbrains.kotlin.gradle.dsl.JvmTarget.JVM_17
           }
       }
   }

Reproduction

• Flutter 3.44.1, AGP 9.2.1, Gradle 9.4.1
• Run flutter clean && flutter build apk
• Observe: FAILURE … Could not find method kotlin() for arguments on build.gradle:58

After this fix the build succeeds with only a deprecation warning
(expected until downstream projects fully migrate to built-in Kotlin).