Make demo app functional without an account

> Maybe we should create a fallback demo app ticket for iOS & Android. Just so devs can use the simulated route w/ demo tiles without having to sign up to stadia and get a key. Obviously this would be reduced demo functionality. While it's not a huge hurdle, it would keep the minimum requirements to contribute a little lower.

_Originally posted by @Archdoog in https://github.com/stadiamaps/ferrostar/pull/267#discussion_r1776107478_

Ideally we can figure out how to make the demo apps on both iOS and Android automatically degrade gracefully to some demo tiles (MapLibre demo? Americana?) and the FOSSGIS server so that, even if you don't enter an API key, the app "just works" out of the box.

- [ ] iOS
- [ ] Android

Start by locating the iOS and Android demo app entry points and their API-key configuration. Check how each app selects map tiles when no key is present, then verify that both apps launch with demo tiles and the FOSSGIS server without requiring an account.

Written by the indexing model from the issue text.