Upgrading MvvmCross to 7.1.2: XF App throws exception on Android when using DatePicker
## 🔙 Regression

<!--- Summary description of the regression --->
Upgrading MvvmCross to 7.1.2: XF App throws exception on Android when using DatePicker

### Current behavior
We used our XF app with MvvmCross 5.xx without problems.
We updated to 7.1.2 (and also XF to latest), and it throws on Android. 
iOS is still ok.

### Reproduction steps
Just call `picker.Focus();`
Exception: Android.Views.WindowManagerBadTokenException: 'Unable to add window -- token null is not valid; is your activity running?'

### Configuration

**Version:** 7.1.2

**Platform:** 
XF on Android

**Please note** 
It has been also reported to Xamarin Forms, where **additional info is also provided**:
https://github.com/xamarin/Xamarin.Forms/issues/8979