[native] Don't redirect to new registration flow if not enabled

Summary:
Realized that in D10412, I set doNotRegister: true for the "main" SIWE login, which due to D10415 will cause us to open up the new registration flow before it's enabled.

In this diff I make sure we don't set doNotRegister: true unless the new registration flow is enabled.

I also introduce a new enableNewRegistrationMode constant, so that we can enable the new registration flow from one place.

Test Plan:

1. Flip enableNewRegistrationMode to false
2. Try using the main SIWE button to register a new user
3. Confirm it succeeds and doesn't redirect to the new registration flow

Depends on D10418

Reviewers: atul, ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10431