urls should always reverse to /firefox/, regardless of UA or ?app= param

[eviljeff]

addons-server part of mozilla/addons#16398

Description

Drop UA detection and support for ?app=android in urls for returning /android/ in a frontend url.

Context

In mozilla/addons-frontend#14469 we stop directing users to /android/ urls (because /firefox/ urls are now functionally the same on Android), but the api still returns /android/ urls when the app param is android, or the UA is detected as Android. This patch removes that functionality too so all urls are returned as /firefox/

Testing

• call /api/v5/addons/addon/<some_slug>/?app=android and see in the detail response that url now ignores the app param and always returns /firefox/.
• repeat without the app param, but with an android user agent defined.
• app should otherwise still be used for compatibility filtering for /search, etc.

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.