Use flavorDimensions for external demo app

- This is soon becoming mandatory.
- It also looks like future versions of com.android.tools.build
  are being distributed via Google's Maven repository.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=166058299

build.gradle

@@ -14,6 +14,9 @@
 buildscript {
     repositories {
         jcenter()
+        maven {
+            url "https://maven.google.com"
+        }
     }
     dependencies {
         classpath 'com.android.tools.build:gradle:2.3.3'

demos/main/build.gradle

@@ -39,9 +39,15 @@ android {
         disable 'MissingTranslation'
     }
 
+    flavorDimensions "extensions"
+
     productFlavors {
-        noExtensions
-        withExtensions
+        noExtensions {
+            dimension "extensions"
+        }
+        withExtensions {
+            dimension "extensions"
+        }
     }
 }