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.cpp b/Interface.cpp
index d453986..9d1adf8 100644
--- a/Interface.cpp
+++ b/Interface.cpp
@@ -25,6 +25,10 @@
     return true;
 }
 
+bool Interface::isBinder() const {
+    return true;
+}
+
 const std::vector<Method *> &Interface::methods() const {
     return mMethods;
 }