Move remaining jobject related functions to use ObjPtr

Also added ObjPtr::DownCast.

Bug: 31113334

Test: test-art-host

Change-Id: I59c253211dc435579ffdfd49f856861ab13d262c
diff --git a/runtime/check_jni.cc b/runtime/check_jni.cc
index 4dc7b31..8a51dc2 100644
--- a/runtime/check_jni.cc
+++ b/runtime/check_jni.cc
@@ -772,7 +772,7 @@
         okay = false;
       } else {
         obj = soa.Vm()->DecodeWeakGlobal(soa.Self(), ref);
-        okay = Runtime::Current()->IsClearedJniWeakGlobal(obj.Ptr());
+        okay = Runtime::Current()->IsClearedJniWeakGlobal(obj);
       }
       if (!okay) {
         AbortF("%s is an invalid %s: %p (%p)",