[native] [12/34] ReactNav6: use NavHelpers for type parameter name instead of NavProp

Summary:
Following the changes in the last three diffs, we want to only use NavProp for actual navigation props (corresponding to a specific RouteName), and instead use the name NavHelpers for cases where we can't infer a specific RouteName.

Depends on D5404

Test Plan: Flow + ran the app on my phone and played around with it

Reviewers: atul, tomek, bartek

Reviewed By: atul

Subscribers: abosh

Differential Revision: https://phab.comm.dev/D5405