add missing android plugin dependency

Due to our unusual project layout, the android/app project is not
considered a child of android/ by Gradle. Therefore, the allprojects and
buildscript blocks in the parent project were not inherited by the
child.

We work around this by simply copying them into the child.

Change-Id: Ibd773cf84da36dd2df3bb7d0db1273a92538a514
diff --git a/android/app/build.gradle b/android/app/build.gradle
index 62a9388..4e5b9ac 100644
--- a/android/app/build.gradle
+++ b/android/app/build.gradle
@@ -5,6 +5,7 @@
 
     dependencies {
         classpath 'com.jakewharton.sdkmanager:gradle-plugin:0.12.+'
+        classpath 'com.android.tools.build:gradle:1.3.0'
     }
 }
 
@@ -30,6 +31,10 @@
     }
 }
 
+repositories {
+    mavenCentral()
+}
+
 dependencies {
     compile fileTree(dir: 'libs', include: ['*.jar'])
     testCompile 'junit:junit:4.12'