[native] Handle unsaved changes when 'cancel' is pressed

Summary:
As a second part to the header left button, we want to ensure that a user is prompted with an alert warning them that they are discarding changes if they've selected a different role but not saved.

Depends on D8158

Test Plan:
Please see the video below

Reviewers: atul, ginsu, tomek

Reviewed By: atul

Subscribers: ted, ashoat

Differential Revision: https://phab.comm.dev/D8159