Use Android Gradle Plug-in v2.2.0-alpha6

Bug: 30104181
Change-Id: Id89054155375caabe3a413cf3ca576d88b3b0673
(cherry picked from commit c1e50455a755588144826141228d52cf6785768e)
diff --git a/build.gradle b/build.gradle
index cf0184c..08e2d60 100644
--- a/build.gradle
+++ b/build.gradle
@@ -9,7 +9,7 @@
         maven { url "../../prebuilts/maven_repo/android" }
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:2.1.2'
+        classpath 'com.android.tools.build:gradle:2.2.0-alpha6'
     }
 }