[native] Use Arial for all body text in registration flow

Summary:
See diff comment here and screenshots in test plan. Basically, using Arial "1." has the same width as "2.", which makes lists look better. We use that trick for Markdown lists, and since we have a couple lists in the registration flow, I want to use the same trick here.

For consistency, I'm updating all of the "body" text in the registration flow to use Arial as well.

Depends on D7925

Test Plan:

before	after

Reviewers: atul, ginsu, ted

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7926