Fixed the Android Playground build

This also reveals a strange issue when starting the Playground app, it throws a

    NotSupportedException: "Unsupported expression type: 'Convert'"

at https://github.com/reactiveui/ReactiveUI/blob/master/ReactiveUI/Platform/Registrations.cs#L46

that I can also reproduce in my apps that I upgraded to RxUI 6
Hm, this should be equivalent