IMvxAndroidCurrentTopActivity: returns last seen activity if no activity exists
## 🐛 Bug Report

### Expected behavior
MvxCurrentTopActivity.Activity returns null if no activity exists.

### Reproduction steps
1. Press back on the main activity.
2. Activity and task will be destroyed. No other activities or tasks exist in the Recents screen.
3. IMvxAndroidCurrentTopActivity.Activity (MvxCurrentTopActivity) returns last seen activity (the main activity).


Just [two lines of code ](https://github.com/MvvmCross/MvvmCross/issues/4334#issuecomment-1099052375) is missing to make it work.

### Configuration

**Version:** 9.0.1

**Platform:** 
- [ ] :robot: Android
