Clean up Class::GetDirectInterface().

Once the `klass` has been resolved, this function should not
need to resolve interface types anymore. Drop the type
resolution from this function and pass the Class as ObjPtr<>
instead of Handle<>. Make callers that expect non-null
result DCHECK() that assumption. For the callers that may
actually need to resolve these interface classes, introduce
a new function ResolveDirectInterface().

Also improve ObjPtr<> constructors and assignment operator
to improve overload resolution. For example, if we have
foo(ObjPtr<T1>) and foo(ObjPtr<T2>), calling foo(.) with
a T1* would have previously been ambiguous even if T1 is
not a base of T2.

Test: m test-art-host
Change-Id: Iff44a7285f68f0af4507fc9ba37343865422fe84
diff --git a/runtime/openjdkjvmti/ti_heap.cc b/runtime/openjdkjvmti/ti_heap.cc
index 5e588a8..5f18b7c 100644
--- a/runtime/openjdkjvmti/ti_heap.cc
+++ b/runtime/openjdkjvmti/ti_heap.cc
@@ -484,7 +484,7 @@
     art::Handle<art::mirror::Class> h_klass(hs.NewHandle<art::mirror::Class>(klass));
     for (size_t i = 0; i < h_klass->NumDirectInterfaces(); ++i) {
       art::ObjPtr<art::mirror::Class> inf_klass =
-          art::mirror::Class::GetDirectInterface(self, h_klass, i);
+          art::mirror::Class::ResolveDirectInterface(self, h_klass, i);
       if (inf_klass == nullptr) {
         // TODO: With a resolved class this should not happen...
         self->ClearException();