Identify cases where same appid doesn't mean same app

Lots of people are confused when having gapps installed and seeing unifiedNlp listed as installed. This will mostly be harmless since we'll disallow uninstalling system apps and once we don't show updates because of the differing signatures.

But it's still wrong to list it under installed. We can probably have a heuristic to detect these. Some ideas:

• If the installed is a system app and the signatures differ, it's a different app
• If the above has too many false positives, try heuristics involving app name or version codes

There are multiples issues about this on fdroiddata. See fdroiddata#569 (closed) for example.