[Android] Update gradle plugin version for Android Studio 3 beta 2

android/build.gradle

         maven { url 'https://maven.google.com' }
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.0.0-alpha9'
+        classpath 'com.android.tools.build:gradle:3.0.0-beta2'
 
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files.