Disable instant run in your Android Studio and Take a build once more. It will work fine.
Now, As per stable available version 2.1.2 of Android studio,
If you need to turn off Instant Run, go to
File → Settings → Build, Execution, Deployment → Instant Run and uncheck Enable Instant Run.
This occurs because The Apk provided for testing may be an instant apk. So Disable it, Clean it and Build it once more. If its not a code issue it will work.
solved My android app is not opening but installing [closed]