We want to type setParams correctly for both the specific-screen and any-screen-on-a-navigator case. As a result, the type of setParams is incompatible between the two.
Note that I have left off setParams from NavigationHelpers in this diff. That's because I need to make some changes to our codebase before I can do that without leading to Flow errors... otherwise the two setParams will collide. I will reintroduce setParams to NavigationHelpers in a later diff in the stack.
Depends on D5401