[Offline] does not work properly on Android

[mariokeusen]

Usecase: open app with an android client and chrome browser, download a perimeter for offline use. Set device to offline mode, reopen app
Result: no map tiles visible
Expected: map tiles are available for the downloaded perimeter

Was reported via helpdesk today and could be reproduced here.
Thanks for fixing.

[davidoesch]

related to #4763 ?

[pakb]

Could be related, but the tech we use under the hood says it should work with Android (see their doc )
Or at least that's what I understand from what we use (but not having coded the thing myself, I just had a glimpse of these part of the app, we should investigate more if we think this is worth it)

[pakb]

I could add that their doc is 5 years old, which is an eon in web technologies. So take it with a grain of salt...