Remove last using directives from global header scope

Bug: None
Test: Compiles, unittests pass

Change-Id: I66dee2504b28049ecb74526a08345e999d57db8d
diff --git a/generate_java.h b/generate_java.h
index 929d15a..6719030 100644
--- a/generate_java.h
+++ b/generate_java.h
@@ -44,7 +44,7 @@
   using Variable = ::android::aidl::java::Variable;
   using Type = ::android::aidl::java::Type;
 
-  VariableFactory(const string& base); // base must be short
+  VariableFactory(const std::string& base); // base must be short
   Variable* Get(const Type* type);
   Variable* Get(int index);