IMvxAndroidCurrentTopActivity incorrectly works for Activity styled as dialog
## Bug Report

Hello
I'm attaching sources which reproduce the problem.

Assume we have flow:
- `MainActivity `opens `DialogFromActivity` (activity styled as dialog)
- `DialogFromActivity` opens `NormalActivity`

Then on return from `NormalActivity`, there is visible `DialogFromActivity`.
Problem is that although app shows `DialogFromActivity` but `IMvxAndroidCurrentTopActivity` claims that current activity is
- sometimes `MainActivity` (bug!)
- sometimes `DialogFromActivity` (ok)

It affects other mechanisms in MvvmCross.
Especially NavigationService - in such cases I get problem AndroidPresenter:
_"Ignoring close for viewmodel - rootframe's current page is not the view for the requested viewmodel"_


### Reproduction steps

Open attached solution (it's based on TipCalc)
- click "Start PoC"
- click "Open dialog"
- click "Open normal activity"
- click "Close"
- in dialog, click "Do test"
- check in logs if there is 
```
    current activity = MainActivityView
    !!!!!!!!!! ERROR !!!!!!!!!!
```
- if no -> close dialog and main activity and try again
- if yes -> bingo, you that's wrong behavior


### Configuration

**Version:** 6.4.2, 7.1.2

I found problem in 6.4.2 but newest MC is also affected.
Can you fix it?
Big thanks! ;]

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

[TipCalc.zip](https://github.com/MvvmCross/MvvmCross/files/6385288/TipCalc.zip)
