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/java_lang_String.cc b/runtime/native/java_lang_String.cc
index b3a967d..5a49c20 100644
--- a/runtime/native/java_lang_String.cc
+++ b/runtime/native/java_lang_String.cc
@@ -41,7 +41,7 @@
     return -1;
   } else {
     return soa.Decode<mirror::String>(java_this)->CompareTo(
-        soa.Decode<mirror::String>(java_rhs).Decode());
+        soa.Decode<mirror::String>(java_rhs).Ptr());
   }
 }