Find your chat profile. Click on the prefs tab (look on the top right of the page)
You should see this...
If you uncheck the pref you will likely need to refresh the page to get the new interface. You may also need to wait a few minutes in case there's any caching going on.
It's not just Android; the mobile web version doesn't have this feature on iOS either. You can get the mobile version on your desktop browser too, by clicking the 'Mobile' link in the footer, and then it doesn't work either.
Fortunately, the feature is enabled in the new responsive design, and it works on desktop, iOS and Android alike. (To switch to this ...
Well, yeah that was broken.
It turns out that the cookie we used to track whether you wanted to use the mobile UI expired on Jan 1st, 2020...so that's why it stopped working:
This was just fixed to extend the expiration until Jan 1st, 2030, so on Jan 2nd, 2030 it will stop working again.