[MaterialDatePicker] Make the class open*.

Nobody has claimed this yet.

**Is your feature request related to a problem? Please describe.**
I would suggest the class to be ~made public~ open, so we could attach the selection listener in a subclass and use "sharedViewModel" to deliver the selected date. At the moment in case of screen rotation, we need to detach the listeners and reattach when the new fragment is ready to avoid a memory leak.

**Describe the solution you'd like**
Make the class public.

**Describe alternatives you've considered**
Maybe another way to avoid subclassing it would be the old fashion OnInteractionListener on the parentFragment / activity.

We also happily accept [pull requests](https://github.com/material-components/material-components-android/pulls).

Start at the MaterialDatePicker entry point and review how selection listeners are currently attached across fragment recreation. Compare the requested subclassing approach with the proposed parentFragment or activity listener alternative. Done means the supported API clearly enables the intended listener and sharedViewModel use without requiring manual listener detachment during rotation.

Written by the indexing model from the issue text.