Remove custom splash screen option

[mhsmith]

Starting in Android 12, the system displays a default splash screen with the app icon, which overrides the previous theme-based technique we were using. Since this is now standard behavior on Android, I think it's probably best to abandon custom splash screens on this platform.

In fact, since iOS is the only other platform that supports splash screens, I don't think it's worth keeping customizable splash screens in Briefcase at all. For iOS, and older versions of Android, both of which display a blank splash screen if the app doesn't provide one, we could match the new Android behavior by auto-generating a splash screen with the app icon.

In their current recommended dimensions, app icons might not be high resolution enough for a splash screen. We could solve that at the same time as #386 by generating all icons and splash screens from a single image file provided by the user.