Allow to build Java module from mere static Java libraries.

With this change, we can easily turn a static Java library to a shared
Java library.

Change-Id: I9915992cd19879846511d6272483d7a0a934e7eb
diff --git a/core/java.mk b/core/java.mk
index 0cc0762..47f7b41 100644
--- a/core/java.mk
+++ b/core/java.mk
@@ -5,7 +5,7 @@
 
 # Make sure there's something to build.
 # It's possible to build a package that doesn't contain any classes.
-ifeq (,$(strip $(LOCAL_SRC_FILES)$(all_res_assets)))
+ifeq (,$(strip $(LOCAL_SRC_FILES)$(all_res_assets)$(LOCAL_STATIC_JAVA_LIBRARIES)))
 $(error $(LOCAL_PATH): Target java module does not define any source or resource files)
 endif
 
@@ -220,7 +220,7 @@
 # If the module includes java code (i.e., it's not framework-res), compile it.
 full_classes_jar :=
 built_dex :=
-ifneq (,$(strip $(all_java_sources)))
+ifneq (,$(strip $(all_java_sources)$(full_static_java_libs)))
 
 # If LOCAL_BUILT_MODULE_STEM wasn't overridden by our caller,
 # full_classes_jar will be the same module as LOCAL_BUILT_MODULE.