Empty page when using MvxPopPresentationHint
## 🐛 Bug Report

I am using MvxPopPresentationHint in order to navigate backward but instead, it pops the current view and shows an empty page on Android platform.

### Expected behavior Navigate to the previous view and show its content.

### Reproduction steps
1. Create two MvxContentPage with the corresponding MvxViewModel attached to them.
2. Add a button on the first page that navigates to the second one using this code:
`await _navigationService.Navigate(new SecondPageViewModel(this.navigationService));`
3. Add a button to the second page that pops it using this code:
`await _navigationService.ChangePresentation(new MvxPopPresentationHint(typeof(FirstPageViewModel)));`
When you navigate from the first page to the second and then pop it, it navigates back to the first one without showing its content.
Please find attached a sample project in which the issue is reproduced.
[MyXamarinFormsApp.zip](https://github.com/MvvmCross/MvvmCross/files/7137564/MyXamarinFormsApp.zip)

### Configuration

**Version:** 6.4.2

**Platform:** 
- [ ] :robot: Android
- [ ] :monkey: Xamarin.Forms
