code: public static Session openActiveSession(Context context, Fragment fragment,
              boolean allowLoginUI, StatusCallback callback) {
          return openActiveSession(context, allowLoginUI, new OpenRequest(fragment).setCallback(callback));
      }
docs: If allowLoginUI is true, this will create a new Session, make it active, and
  open it. If the default token cache is not available, then this will request
  basic permissions. If the default token cache is available and cached tokens
  are loaded, this will use the cached token and associated permissions.
  <p/>
  If allowedLoginUI is false, this will only create the active session and open
  it if it requires no user interaction (i.e. the token cache is available and
  there are cached tokens).
  
  @param context      The Activity or Service creating this Session
  @param fragment     The Fragment that is opening the new Session.
  @param allowLoginUI if false, only sets the active session and opens it if it
  does not require user interaction
  @param callback     The {@link StatusCallback SessionStatusCallback} to
  notify regarding Session state changes.
  @return The new Session or null if one could not be created