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/gc/heap.cc b/runtime/gc/heap.cc
index b4c2d14..ef31be3 100644
--- a/runtime/gc/heap.cc
+++ b/runtime/gc/heap.cc
@@ -2765,7 +2765,7 @@
 void Heap::CheckPreconditionsForAllocObject(mirror::Class* c, size_t byte_count) {
   CHECK(c == NULL || (c->IsClassClass() && byte_count >= sizeof(mirror::Class)) ||
         (c->IsVariableSize() || c->GetObjectSize() == byte_count) ||
-        strlen(ClassHelper(c).GetDescriptor()) == 0);
+        c->GetDescriptor().empty());
   CHECK_GE(byte_count, sizeof(mirror::Object));
 }