Enable Kotlin for RN Tester (facebook#38791)

Summary:
Pull Request resolved: facebook#38791

This is needed if we want to allow users to write Kotlin code inside RN Tester

Changelog:
[Internal] [Changed] - Enable Kotlin for RN Tester

Reviewed By: cipolleschi

Differential Revision: D48066316

fbshipit-source-id: 63c820b3b96899b8f30d11176e9ea6201e0bef17

packages/rn-tester/android/app/build.gradle

@@ -8,6 +8,7 @@
 plugins {
     id("com.android.application")
     id("com.facebook.react")
+    id("org.jetbrains.kotlin.android")
 }
 
 /**