Cleaned up use of substr() to get interface's base name.

Bug: 31527442

Test: make at top-level. hidl_test passes.

Change-Id: Ia940a421599dc158e26fb7ea481abeab32620f65
diff --git a/generateVts.cpp b/generateVts.cpp
index fe71487..c830918 100644
--- a/generateVts.cpp
+++ b/generateVts.cpp
@@ -85,7 +85,8 @@
         baseName = "types";
         isInterface = false;
     } else {
-        baseName = ifaceName.substr(1);  // cut off the leading 'I'.
+        const Interface *iface = mRootScope->getInterface();
+        baseName = iface->getBaseName();
     }
 
     path.append(baseName);