Fix proguard flag file dependency.

Change-Id: I9a41043a4dc44f6d46f19318ee3baa1bd1b87bb2
diff --git a/Android.mk b/Android.mk
index 4cb1edb..2abde9b 100644
--- a/Android.mk
+++ b/Android.mk
@@ -12,7 +12,7 @@
 
 LOCAL_PACKAGE_NAME := Browser
 
-LOCAL_PROGUARD_FLAGS := -include $(LOCAL_PATH)/proguard.flags
+LOCAL_PROGUARD_FLAG_FILES := proguard.flags
 
 LOCAL_EMMA_COVERAGE_FILTER := *,-com.android.common.*