Support for a generic "binder" type (reflected as "interface" in hidl)

marshaled as a sp<hwbinder::IBinder>.

Bug: 30884691
Change-Id: I157d78fbbe839637805359441925aec597c52383
diff --git a/Interface.h b/Interface.h
index c74bbfe..f61598a 100644
--- a/Interface.h
+++ b/Interface.h
@@ -21,6 +21,7 @@
     void addMethod(Method *method);
 
     bool isInterface() const override;
+    bool isBinder() const override;
 
     const Interface *superType() const;