Duplicate OnCreate on startup when theme is selected

## 🐛 Bug Report
When a theme is selected, MainActivity.OnCreate is called twice upon startup. That results in 2x initialization, and end result is that the RootPage will show a back button, which when tapped, goes back to an empty tab page.

### Expected behavior
Don't initialize app twice when a theme is selected.

### Reproduction steps
1. Select a different theme than "Auto"
2. Close app (properly)
3. Open app again.
3.a. Back button should now be showing
4. Tap back button
5. Now you're stuck on an empty tab page, without being able to navigate further in the app. You have to close it and reopen.

### Configuration
I only tested Android. Might be an issue on other platforms too.

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

Start at the Android MainActivity.OnCreate entry point and reproduce the issue by selecting a non-Auto theme, closing the app, and reopening it. Trace why startup initialization occurs twice and verify that the RootPage no longer shows a back button or leads to an empty tab page.

Written by the indexing model from the issue text.