gerrit-app-reviewit: Ensure the ~/.android directory exists

This works around the following bug in the Android Gradle plugin:

https://code.google.com/p/android/issues/detail?id=223424

Change-Id: I70930b74d06eb021f99e59489fdf634101cd18c9
diff --git a/jenkins/gerrit-app-reviewit.yaml b/jenkins/gerrit-app-reviewit.yaml
index 63f4875..c2f875f 100644
--- a/jenkins/gerrit-app-reviewit.yaml
+++ b/jenkins/gerrit-app-reviewit.yaml
@@ -4,5 +4,5 @@
       - master
     jobs:
       - 'app-{name}-{branch}'
-    commands: './gradlew assemble'
+    commands: 'mkdir -p ~/.android && ./gradlew assemble'
     artifacts: 'app/build/outputs/apk/*.apk'