Enable proguard

Enable proguard for optimization (not obfuscation) to strip all of
the unnecessary methods, such as the majority of Guava.

Change-Id: Ic632ff6ae3b1a4c058090d843937867b3cde4447
diff --git a/Android.mk b/Android.mk
index 6897fe0..76a190e 100644
--- a/Android.mk
+++ b/Android.mk
@@ -13,6 +13,8 @@
 LOCAL_CERTIFICATE := platform
 LOCAL_PRIVILEGED_MODULE := true
 
+LOCAL_PROGUARD_ENABLED := optimization
+
 include $(BUILD_PACKAGE)
 
 # Build the test package.