unity 5.6 cardboard - how to use back button for a custom action

I've been trying to use the Cardboard back button (on screen , top left x) to exit VR mode. Before the native unity integration (5.5) I was able to just use gvrViewer.BackButtonPressed but this doesn't seem to work now.
I've also tried if (Input.GetKeyDown(KeyCode.Escape)) but the app still exits.
So I'm added a Input.backButtonLeavesApp = false; but it...

unity 5.6 cardboard - how to use back button for a custom action