## Description

This adds `AppLifecycleListener`, a class for listening to changes in the application lifecycle, and responding to requests to exit the application.

It depends on changes in the Engine that add new lifecycle states: https://github.com/flutter/engine/pull/42418

Here's a diagram for the lifecycle states. I'll add a similar diagram to the documentation for these classes.

![Application Lifecycle Diagram](https://github.com/flutter/flutter/assets/8867023/f6937002-cb93-4ab9-a221-25de2c45cf0e)

## Related Issues
 - https://github.com/flutter/flutter/issues/30735

## Tests
- Added tests for new lifecycle value, as well as for the `AppLifecycleListener` itself.