[ion] Update the Event model

Change-Id: I41a53caeb7dd8a15e7934f49675f6a53be8fe7be

apps/graph/values/function_parameter_controller.cpp

@@ -38,12 +38,10 @@ void FunctionParameterController::didBecomeFirstResponder() {
 }
 
 bool FunctionParameterController::handleEvent(Ion::Events::Event event) {
-  switch (event) {
-    case Ion::Events::Event::ENTER:
-      return handleEnter();
-    default:
-      return false;
+  if (event == Ion::Events::OK) {
+    return handleEnter();
   }
+  return false;
 }
 
 bool FunctionParameterController::handleEnter() {