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_DexFile.cc b/runtime/native/dalvik_system_DexFile.cc
index 0677d5b..086da60 100644
--- a/runtime/native/dalvik_system_DexFile.cc
+++ b/runtime/native/dalvik_system_DexFile.cc
@@ -287,7 +287,7 @@
                                                         *dex_class_def);
       // Add the used dex file. This only required for the DexFile.loadClass API since normal
       // class loaders already keep their dex files live.
-      class_linker->InsertDexFileInToClassLoader(soa.Decode<mirror::Object>(dexFile).Decode(),
+      class_linker->InsertDexFileInToClassLoader(soa.Decode<mirror::Object>(dexFile).Ptr(),
                                                  class_loader.Get());
       if (result != nullptr) {
         VLOG(class_linker) << "DexFile_defineClassNative returning " << result