Extend hidl-gen to support generate vts file.

b/30762234

TODO: add regression test.

Change-Id: I1c31fd9a85805cd450ea03cc0ccc750a756d1009
diff --git a/ScalarType.h b/ScalarType.h
index 3e84597..a083d54 100644
--- a/ScalarType.h
+++ b/ScalarType.h
@@ -51,6 +51,8 @@
             ErrorMode mode,
             bool needsCast) const;
 
+    status_t emitVtsTypeDeclarations(Formatter &out) const override;
+
 private:
     Kind mKind;