Feching facebook session from default facebook

I am developing following requirements for android app-

1. Check if Facebook session is available (if user is logged into his default facebook app of android device).
2. If he has session fetch token from it and use graph api.
3. If not show him error that he should login to default facebook app.

In my app there is only service and no activity, so I don't want to show activity to user to enter his facebook credentials and then use graph apis. I WANT TO FETCH SESSION FROM DEFAULT FACEBOOK APP