Android navigation: add a way to pop the root

[sdetilly]

This functionality becomes necessary when the Pilot navigation stack is not implemented at the app's root level but is instead utilized within a specific app section. Given that the navigation stack might be integrated within a navigation controller or displayed in a modally, the responsibility to close the stack must be transferred to the caller.

To address this requirement, an optional closure named "onRootPop" has been introduced as an override for PilotNavControllerNavigationListener, enabling a flexible approach to close the stack.

[npresseault]

On iOS, that is handled with a closure on the Navigation object instead but I think subclassing is a good option here for that use case. I don't object to that API.