Delete ClassHelper and fix compaction bug in GetDirectInterface

Cleanup helps to prevent compaction bugs. Fixed a fairly serious
compaction error caused by calling ClassHelper::GetDirectInterface
without handling the case where it causes thread suspension due to
ResolveType.

Bug: 8981901

Change-Id: I82b3bb6dd48d21eb6ece7aae0733c4a23c2bc408
diff --git a/runtime/native/java_lang_Class.cc b/runtime/native/java_lang_Class.cc
index b6cf7d8..e619dda 100644
--- a/runtime/native/java_lang_Class.cc
+++ b/runtime/native/java_lang_Class.cc
@@ -84,8 +84,9 @@
 
 static jstring Class_getNameNative(JNIEnv* env, jobject javaThis) {
   ScopedFastNativeObjectAccess soa(env);
-  mirror::Class* c = DecodeClass(soa, javaThis);
-  return soa.AddLocalReference<jstring>(c->ComputeName());
+  StackHandleScope<1> hs(soa.Self());
+  mirror::Class* const c = DecodeClass(soa, javaThis);
+  return soa.AddLocalReference<jstring>(mirror::Class::ComputeName(hs.NewHandle(c)));
 }
 
 static jobjectArray Class_getProxyInterfaces(JNIEnv* env, jobject javaThis) {