Rename ObjPtr::Decode to ObjPtr::Ptr

Done to prevent ambiguity with ScopedObjectAccess::Decode.

Bug: 31113334

Test: test-art-host
Change-Id: I07a2497cc9cf66386311798933547471987fc316
diff --git a/runtime/native/dalvik_system_VMDebug.cc b/runtime/native/dalvik_system_VMDebug.cc
index 73c4664..8e81bc9 100644
--- a/runtime/native/dalvik_system_VMDebug.cc
+++ b/runtime/native/dalvik_system_VMDebug.cc
@@ -263,7 +263,7 @@
   if (c == nullptr) {
     return 0;
   }
-  std::vector<mirror::Class*> classes {c.Decode()};
+  std::vector<mirror::Class*> classes {c.Ptr()};
   uint64_t count = 0;
   heap->CountInstances(classes, countAssignable, &count);
   return count;