Cannot update segment list of "Segment" type

[zackcl]

Version where bug was found:
6.1.0

Describe the bug
When trying to update a segment list of "Segment" type (private segment), the backend returns 400 Bad Request error, saying the segment with same name already exists for this app-context. This also happens when trying to update a list of other types (e.g., Individual) to Segment type on the "Edit List" modal.

To Reproduce
Steps to reproduce the behavior:

1. Create a new segment
2. Go to the details page
3. Add a list of type "Segment" and select any available segment
4. Click the edit icon to open the "Edit List" modal
5. Change the segment to something else and click "Save"
6. See that a 400 Bad Request error is returned from the backend

Expected behavior
We should allow updating a segment list of "Segment" type to use different segment without the error. We should also allow updating a segment list of other types (e.g., Individual) to Segment type without the error.