System.InvalidCastException getting MvxFragment<TViewModel>.ViewModel after Tombstoning
## 🐛 Bug Report

<!--- Summary description of the bug --->
Fragments will not have a valid ViewModel after tombstoning the app. The data context of the fragment will consistently be MvxNullViewModel. Accessing the ViewModel property of the fragment will trigger an InvalidCastException.

### Expected behavior
View models should be restored / recreated, and the fragments should have their ViewModel property set to the correct view model.

### Reproduction steps
Have an MvvmCross app with an MvxFragment<TViewModel> that accesses the ViewModel property in OnResume.
1. Launch the app, and have the fragment of type MvxFragment<TViewModel> active.
2. Press the home button to leave the app.
3. Flood the device memory. [Developer Toolkit](https://play.google.com/store/apps/details?id=com.tspoon.androidtoolbelt) is useful for this.
4. Return to the app after it is tombstoned. It will crash due to System.InvalidCastException.

### Example Project Repo
https://github.com/MagneticLlama/MvxTombstoneFragments

### Configuration

**Version:** 9.x

**Platform:** 
- [ ] :iphone: iOS
- [x] :robot: Android
- [ ] :checkered_flag: WPF
- [ ] :earth_americas: UWP
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [ ] :monkey: Xamarin.Forms
