[native] [28/34] ReactNav6: Specify headerShown: false for tab bar

Summary:
Context on change here.

Depends on D5420

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/D5421