[native] Make QR screen style consistent with the registration flow

Summary:
Avoid using hardcoded margin and make the styling consistent with the registration flow. In the following diffs new screens from the restoration flow are introduced and all of these should follow the same theme.

https://linear.app/comm/issue/ENG-8129/qr-code-design-doesnt-look-consistent-with-the-rest-of-the-app

Depends on D13906

Test Plan: Check if the screen looks correct.

Reviewers: kamil, angelika, bartek, ashoat

Reviewed By: ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D14064