BindingContext not set on MvxActivity
## 🔙 Regression

<!--- Summary description of the regression --->

### Old (and correct) behavior

BindingContext was set when OnCreate would be called

### Current behavior

This behavior is broken

### Reproduction steps

Just start playground, you will see all the buttons being disabled

### Configuration

**Version:** 8.0.0

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