Replace MvxSetup and related code with Microsoft.Extensions.Hosting
The trend in the .NET ecosystem looks to be to implement Microsoft.Extensions.Hosting to build the Application, setup various services, setup fonts (MAUI) and much more.

We should look into replacing MvxSetup and reworking the startup procedure of MvvmCross and adopt Microsoft.Extensions.Hosting.

While working on this, we should ditch MvxSplashScreen on Android, and move all startup to an instance of Application.

- [ ] Remove MvxSplashScreen
- [ ] Replace MvxSetup with implementation of Microsoft.Extensions.Hosting