Update hidl-gen support for vts.

* Support the new Enum type (based on scalar_data)
* Support sub_struct/sub_union defined within compound type.
* Code cleanup: use getVtsType() instead of hard code ones.

Test: make hidl-gen, locally run make hidl_gen_test.
Bug: 30762234
Change-Id: I9a21b5757e0a9fc6cd1bf829ab123565a7990ad5
diff --git a/EnumType.h b/EnumType.h
index 1e6a334..b4d0b15 100644
--- a/EnumType.h
+++ b/EnumType.h
@@ -52,6 +52,8 @@
 
     std::string getJavaWrapperType() const override;
 
+    std::string getVtsType() const override;
+
     void emitReaderWriter(
             Formatter &out,
             const std::string &name,