Restore rules.gradle.

Change-Id: Ie1f3aeed20e9f199836b2ced016515893af26c11
diff --git a/rules.gradle b/rules.gradle
new file mode 100644
index 0000000..1ec9ee4
--- /dev/null
+++ b/rules.gradle
@@ -0,0 +1 @@
+apply plugin: 'com.android.library'