Fix Android build error.

The latest version of Android Studio fails with:

    Could not find com.android.tools.build:aapt2

To use AAPT2, we must now have a dependency on the `google()` repo
in both places in the root build.gradle file, as per:

    https://developer.android.com/studio/releases/#aapt2_gmaven

Author: prideout

android/build.gradle

@@ -6,14 +6,15 @@ buildscript {
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.1.3'
+        classpath 'com.android.tools.build:gradle:3.3.0'
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files
     }
 }
 
 allprojects {
     repositories {
+        google()
         jcenter()
     }
 }