hidl-gen: clean up generateCpp.cpp and generateCppImpl.cpp

Test: hidl_test
Change-Id: I18a9e0b27d40d607dc8b277d9e04eaa6f529f225
diff --git a/Method.h b/Method.h
index 7e711f0..4f5163d 100644
--- a/Method.h
+++ b/Method.h
@@ -62,8 +62,8 @@
     size_t getSerialId() const;
 
     void generateCppSignature(Formatter &out,
-                              const std::string &className,
-                              bool specifyNamespaces) const;
+                              const std::string &className = "",
+                              bool specifyNamespaces = true) const;
 
     static std::string GetArgSignature(const std::vector<TypedVar *> &args,
                                        bool specifyNamespaces);