Make it a fatal error if no source files for Java module.

Change-Id: Ia04158c11381b6b1687b9d5c699a9ea8ae3cb317
diff --git a/core/static_java_library.mk b/core/static_java_library.mk
index c3c5c86..cdb98f0 100644
--- a/core/static_java_library.mk
+++ b/core/static_java_library.mk
@@ -59,6 +59,8 @@
 endif  # all_resources
 endif  # LOCAL_RESOURCE_DIR
 
+all_res_assets := $(all_resources)
+
 include $(BUILD_SYSTEM)/java_library.mk
 
 ifneq (,$(all_resources))