Extend hidl-gen to support generate vts file.

b/30762234

TODO: add regression test.

Change-Id: I1c31fd9a85805cd450ea03cc0ccc750a756d1009
diff --git a/StringType.h b/StringType.h
index 0fc6223..7b78305 100644
--- a/StringType.h
+++ b/StringType.h
@@ -35,6 +35,8 @@
 
     bool needsEmbeddedReadWrite() const override;
     bool resultNeedsDeref() const override;
+
+    status_t emitVtsTypeDeclarations(Formatter &out) const override;
 };
 
 }  // namespace android