Fix proguard flag file dependency.

Change-Id: I0d7a1fb751a536b6e96256f8a65820d1f2074431
diff --git a/Android.mk b/Android.mk
index dce117d..d2b7691 100644
--- a/Android.mk
+++ b/Android.mk
@@ -10,7 +10,7 @@
 
 LOCAL_SDK_VERSION := current
 
-LOCAL_PROGUARD_FLAGS := -include $(LOCAL_PATH)/proguard.flags
+LOCAL_PROGUARD_FLAG_FILES := proguard.flags
 
 include $(BUILD_PACKAGE)