Consider using back_button_interceptor

**Is your feature request related to a problem? Please describe.**

Currently flow_builder implementation has a conflict with packages that use the [back_button_interceptor](https://github.com/marcglasberg/back_button_interceptor) package, because both use the method `SystemChannels.navigation.setMethodCallHandler` to listen to navigation events. This causes the `onGeneratePages` callback to no longer being called on android back button navigation in certain scenarios.

**Describe the solution you'd like**
A clear and concise description of what you want to happen.
As the implementation of back_button_interceptor is lightweight and stable (nearly no open issues, version 6), I propose using it instead of relying on the own interceptor implementation. This resolves any conflicts and results in less code to maintain for flow_builder.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.
Sadly, I was unable to find a solution to merge to calls to the `SystemChannels.navigation.setMethodCallHandler` API. If that is somehow possible, this would also be a viable option.

**Additional context**
Add any other context or screenshots about the feature request here.
I discovered the bug when working with the [loader_overlay](https://github.com/rodrigobastosv/loading_overlay) package. Initially I filled a [bug report there](https://github.com/rodrigobastosv/loading_overlay/issues/33). After discovering the conflict, I filled a bug report with back_button_interceptor [here](https://github.com/marcglasberg/back_button_interceptor/issues/23). The maintainer confirmed the conflict, but declined to work on a possible solution, as they are not using flow_builder.