Add `resetMutableState` method to StateLifecycleManager

Add `resetMutableState` method to StateLifecycleManager

```
/**
* Reset the mutable state to the latest immutable state.
*/
void resetMutableState();
```

Locate StateLifecycleManager and read its existing state-management methods first. Use the issue's signature and documentation as the starting point; done when resetMutableState is added with the requested contract and the relevant project checks pass.

Written by the indexing model from the issue text.