Extend hidl-gen to support generate vts file.

b/30762234

TODO: add regression test.

Change-Id: I1c31fd9a85805cd450ea03cc0ccc750a756d1009
diff --git a/Scope.h b/Scope.h
index 66b11f9..0c3ca68 100644
--- a/Scope.h
+++ b/Scope.h
@@ -37,6 +37,10 @@
     status_t emitTypeDefinitions(
             Formatter &out, const std::string prefix) const override;
 
+    Vector<Type *> getSubTypes() const;
+
+    status_t emitVtsTypeDeclarations(Formatter &out) const override;
+
 private:
     Vector<Type *> mTypes;
     KeyedVector<std::string, size_t> mTypeIndexByName;