Investigate back navigation after opening exercise/lecture link from channel

[julian-wls]

Describe the bug

As mentioned in this conversation of #354 after navigating from a channel to an exercise or a lecture the back button does not work as expected. The behavior is not the same for the Android back button and the top bar back button.

To Reproduce
Steps to reproduce the behavior:

1. Go to a channel that has a corresponding exercise/lecture
2. Click on the info button
3. Click on Go to exercise/lecture
4. Try to navigate back using both the Android back button and the top bar back button
5. See that the behavior is different

Additional context

This issue was introduced with #354.