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/native/java_lang_Class.cc b/runtime/native/java_lang_Class.cc
index 642826c..3341f53 100644
--- a/runtime/native/java_lang_Class.cc
+++ b/runtime/native/java_lang_Class.cc
@@ -278,7 +278,7 @@
 
     uint32_t num_direct_interfaces = h_clazz->NumDirectInterfaces();
     for (uint32_t i = 0; i < num_direct_interfaces; i++) {
-      ObjPtr<mirror::Class> iface = mirror::Class::GetDirectInterface(self, h_clazz, i);
+      ObjPtr<mirror::Class> iface = mirror::Class::ResolveDirectInterface(self, h_clazz, i);
       if (UNLIKELY(iface == nullptr)) {
         self->AssertPendingException();
         return nullptr;