[MaterialInputDialog] Auto focus + close on completion

## 🚀 Feature Requests

*The following discusses my experience on Android, I'm not sure about other platforms.*

I'm happy to submit a small PR to auto focus on the text field when the input dialog opens and hook up the completion event to close the dialog when the user taps on the return key (if the positive button is enabled).

Currently you must tap on the text field manually before entering text, and when you're done you must tap on the positive button. This change would make it so that you can simply start typing straight away, and then press the return key to close the dialog (`Completion` event).

I'm not sure what the side effects of doing this are (and if it's against the Material Design guidelines), so I'm happy to chuck these changes behind an optional flag.

[Current Changes](https://github.com/Baseflow/XF-Material-Library/compare/develop...DominicMaas:DominicMaas/input-dialog)

### Platforms affected (mark all that apply)
- [ ] :iphone: iOS
- [ ] :robot: Android
- [ ] :checkered_flag: WPF
- [ ] :earth_americas: UWP
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [x] :monkey: Xamarin.Forms