Fragment with Tag-Property of MvxFragmentPresentation-Attribute not found using FindFragmentByTag
## 🐛 Bug Report

The Tag-Property of MvxFragmentPresentation-Attribute is not used in MvxAndroidViewPresenter in PerformShowFragmentTransaction. Instead FragmentJavaName is always used as Tag in FragmentTransaction AddToBackStack() and Replace() calls.

### Expected behavior
If in a Fragment the Tag-Property of the MvxFragmentPresentation-Attribute is used, I would expect that I can find the fragment e.g. in BackStack by using FindFragmentByTag. This was the behavior in Version 6.4.1.

### Reproduction steps

1. In Fragment A in MvxFragmentPresentation-Attribute set the tag to e.g. "MyFragmentA"
2. Navigate to Fragment B that has AddToBackStack set to true.
3. In Fragment B call FindFragmentByTag("MyFragmentA") -> Fragment is not found in BackStack

### Configuration

**Version:** 7.1.2

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