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/runtime.cc b/runtime/runtime.cc
index df0dca0..7032565 100644
--- a/runtime/runtime.cc
+++ b/runtime/runtime.cc
@@ -555,7 +555,7 @@
   // We can't run in a transaction yet.
   contextClassLoader->SetObject<false>(
       soa.Self()->GetPeer(),
-      soa.Decode<mirror::ClassLoader>(system_class_loader.get()).Decode());
+      soa.Decode<mirror::ClassLoader>(system_class_loader.get()).Ptr());
 
   return env->NewGlobalRef(system_class_loader.get());
 }