[native] Introduce RegistrationButtonContainer

Summary:
What we really want is the gap style introduced in React Native 0.71.

To make it so we have a consistent 16px above, between, and below buttons, my approach in this diff is:

1. Add a RegistrationButtonContainer that always applies an 8px margin above and below buttons.
2. Make all RegistrationButtons apply an additional 8px margin above and below themselves.

Depends on D7831

Test Plan:
With an artificially tall ConnectEthereum screen:

before	after

Other screens (which only have one button) are unchanged.

Reviewers: ginsu, atul

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7832