Copyright infringement issue google play (An android app)

Sorry to hear your frustrating situation.

Copyright infringement is a very serious issue in business world. Considering the amount of time you have put on your project, it may be the best interest of you to consult with an IP attorney regarding your situation.

Two points here:

First, one of the last documents I signed before I was hired by my previous company(Internet & Software sector) was the paper asking if I was holding any patent, signed NDA, or snippet/library of proprietary code from previous projects. I assume the companies often ask this question to its prospective employee as a preventative measure to avoid nasty IP battles. Thinking conversely, you may want to ask your previous employer if he is currently holding IP-related rights with his application and has substantial evidence(s) that your application has violated his IP. Whether you have been knowingly using his ideas or codes or not, if your previous employer has legal rights on his application, you may find your case very hard to contest.

Second, please let's remind ourselves that reverse-engineering is against the copyright law, unless a person "lawfully obtain[s] the right to use a copy of computer program". And, no, the "hundereds of app of apps which does the same things" may not be strictly following the copyright law. Just because there exist someone else who is getting away from being prosecuted, you don't get an automatic right to imitate his or her wrongdoings.

Disclaimer: I am not an expert in law, and any comment/suggestion I have here is for reference purposes only. I won't be responsible for any legal issue you may encounter caused by my answer.

Copyright does not protect ideas, but rather expressions. In your question, it sounds like the code or design of your app. You can visually compare your app to what is claimed to be protected (registered). Even if it is not an exact duplicate, you may have an issue. You may want to trace the origin of your code. You might want to request clarification from the copyright owner what is registered, and what he believes is infringing.

If the allegation is broader than copying code or visual design of the app, then maybe there is some question about a patent or trade secret. You can ask what trade secret or patent is being asserted.

You might want to speak with an attorney, especially since you rely on income from the app. An attorney will be able to determine the seriousness of the allegation and clear up any misunderstanding. It is probably the fastest route to getting the app back online if the allegation of infringement is incorrect. Good luck.