1488562 - Simplify discovery pane metadata

Status: NEW

(Toolkit :: Add-ons Manager, enhancement, P3)

Description

[Andrew Swan [:aswan]]

We currently have code here that sends a list of installed addons to the disco pane:
https://searchfox.org/mozilla-central/rev/c3fef66a5b211ea8038c1c132706d02db408093a/toolkit/mozapps/extensions/content/extensions.js#1991-2008

The front-end does some horrible hacks to reduce this to 2 bits of information -- whether the user has any (non-built-in) extensions and/or themes installed:
https://github.com/mozilla/addons-frontend/blob/22bdd384e4a39b3e4f80ae5acfdeb67f5cd6f1b5/src/disco/tracking.js

The server-side code here needs to be updated manually and we've neglected to do that recently (e.g., AMO doesn't know that the default theme id changed in 61)  We're also sending a ton of information that is unused.  Let simplify this by just computing hasExtensions and hasThemes in the browser and sending that instead of the full list of installed addons.  This will needs a corresponding server-side change as well...