ART: Rename ArtMethod JNI field

The field is multi-purpose, rename it to data and clean up
accessors in preparation of more checks.

Test: m test-art-host
Change-Id: Ib4b84b7b1a51ca201544bc488ce8770aa858c7fd
diff --git a/runtime/art_method-inl.h b/runtime/art_method-inl.h
index 26450c4..91258c7 100644
--- a/runtime/art_method-inl.h
+++ b/runtime/art_method-inl.h
@@ -503,7 +503,7 @@
       SetEntryPointFromJniPtrSize(new_native_code, pointer_size);
     }
   } else {
-    DCHECK(GetEntryPointFromJniPtrSize(pointer_size) == nullptr);
+    DCHECK(GetDataPtrSize(pointer_size) == nullptr);
   }
   const void* old_code = GetEntryPointFromQuickCompiledCodePtrSize(pointer_size);
   const void* new_code = visitor(old_code);