Add a (dark-mode) splash screen

[BartG95]

Problem: when I start the app, I am first greeted by an almost white splash screen (except for the Orgzly icon).

I can't find it now, but I remember that it is a feature of some recent Android release that Android automatically generates a splash screen if the app doesn't provide it itself. For some reason, Android decides a white splash screen is OK (while both Orgzly and the rest of my phone are in dark-mode).

If I am correct, the solution would be to just add an actual splash screen.

While I am writing this, I stumbled upon this, which might a good first resource (I haven't read it in full, yet): https://developer.android.com/develop/ui/views/launch/splash-screen