Android - How to track app Resumes only when you exit and come back to the app?

I have an issue. For analytic purposes I need to track when the APP (not activity) is resumed. The problem I have now is that if I put the tracker on the OnResume event of an activity, it will get fired every time the user goes back and forth on different activities.

How can I avoid that? How can I track the real "Application Resume," (when user actually exits the app and come back) and not the activity resume?

Any ideas is greatly appreciated. Thanks.