We scan for addon changes twice, once early in startup (usually with no scanning) and once after ui startup.  We hold on to the startup data, and during both scans we restore that data into the addon location instances.  The problem here is if we install a builtin in-between these scans.  The new data from the install would get overwitten by the old data.  In some cases this caused addons to disappear (e.g. old data has incorrect path).  Other issues covered here is that we would never remove addon data for builtins removed from the system, and we would additionally mark builtins as sideloads, which caused other side effects (particularly with search addons) where we would not load the addon, but fortunately the search service later re-installes them.

Differential Revision: https://phabricator.services.mozilla.com/D95422