The app keeps you logged in until you explicitly log out. Tapping the application icon on the device's Home screen will launch the app in the same state as when you minimized the app. If you had logged out, logging in with the same credentials will not start the initial sync again.

In order to launch the app in offline mode, ensure that you do not log out of the app. You can just close the app and click the app icon on the device's Home page to access it in offline mode.