Roll back to gradle 2.1.2 to fix builds.

Change-Id: I5ed3e657a7ddbe642f31c920b2fa3d8035435815
diff --git a/build.gradle b/build.gradle
index 1a3eb36..3d2e800 100644
--- a/build.gradle
+++ b/build.gradle
@@ -13,7 +13,7 @@
         maven { url "../../prebuilts/maven_repo/android" }
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:2.2.0-alpha1'
+        classpath 'com.android.tools.build:gradle:2.1.2'
     }
 }