[native] Introduce RegistrationContainer

Summary:
Factoring out the container and header height logic so it can be reused in other screens. More context in the preceding diff.

Depends on D7500

Test Plan: Everything still works the same as before. Also Flow

Reviewers: ginsu, atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7501