[Android] Reset app start status only when Activity is finishing
## 🐛 Bug Report

Currently when an `Activity` is about to be destroyed it is marked as tearing down and resets app start state: https://github.com/MvvmCross/MvvmCross/blob/9.1.1/MvvmCross/Platforms/Android/Views/MvxActivityViewExtensions.cs#L92
But it is not completely valid scenario on Android in case of low memory activity destroy or with option "Don't keep activities" in "Developer options" because user did not leave the app themself. So then when coming back to the app it gets correctly recreated with all the fragments and navigation stack restored. But then it navigates to the first VM again since app start status was reset and replaces all the already restored fragments and calls their VM initialization again.

### Expected behavior

When coming back to the app that was destroyed by the system (not the user) and then got restored it doesn't navigate to the first VM again.
So checking only `activity.IsFinishing` is needed. Should be `false`.
If the user leaves the app intentionally by clicking Back system button or by swiping it away from Overview then `activity.IsFinishing` is `true` and then after coming back to the app it has to navigate to first VM (current behaviour for all scenarios).
Basically the correct way to fix the issue is to remove checking `activity.IsDestroyed`.

### Reproduction steps

Enable "Don't keep activities" in "Developer options".
Open your MvvmCross app then leave it by clicking Home and come back to the app.

### Configuration

Single activity app.
Not using `MvxStartActivity` since it's an incorrect way to start an app - you must navigate to the first VM that is an `Activity`.
I do call `IMvxAppStart.StartAsync` with condition `IMvxAppStart.IsStarted` (that is always `false` after restoration) from my single `Activity` and it navigates to the first VM that is a `Fragment`, not another `Activity`.

**Version:** 9.1.1

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