CodeListener should not get TreehouseView parameter

[JakeWharton]

There is no way to guarantee the TreehouseView subtype nor the type parameter type that is in use. Instead, the listener should capture the direct TreehouseView subtype instance directly. Through this it does not need a subtype cast, does not need to a type parameter cast, and will actually work with UI systems that do not support arbitrary external mutation (i.e., Compose UI and Swift UI).

[JakeWharton]

Note: This requires some work in our internal usage before it can be done.