BlueBelle is right about how many people will react to the app. That's exactly what will happen. People will get advised multiple times they've been near someone who's infected, but they just can't afford to take time off work or they live in a household where self-isolation is possible.
If this app is going to work, there need to be other measures in place.
People need to be tested and get the result back with very short time, so they know whether they really need to self-isolate and whether they are infected and their own contacts need to self-isolate.
They need to know that there will be no financial loss if they can't work.
In some situations, they might need alternative accommodation, if they can't self-isolate at home.
That would cost money and need efficiency, but I bet it would cost a lot less than the contracts which have already been handed out and there are efficient people out there.
No, Bluebelle I can never agree that the herd immunity idea you're suggesting is a good one or even feasible. BTW This isn't like AIDS at all!