[native] Set urlPrefix in Redux following successful registration

Summary:
Before the successful registration, the previous diffs "hack" server calls to make sure they hit the user's selected keyserver.

Following the successful registration, we can set the urlPrefix in Redux so that all server calls hit the user's selected keyserver.

Depends on D8196

Test Plan:
Tested in combination with the rest of the stack as follows:

1. Override defaultURLPrefix in native/utils/url-utils.js to return some random string. This makes sure that the "default" of Ashoat's keyserver does not work, and makes it so the registration flow only works if the user selects a custom keyserver and all of the server calls in the registration flow use that custom keyserver
2. Test the registration flow end-to-end for an Ethereum account
3. Test the registration flow end-to-end for a username/password account
4. Test the registration flow end-to-end for a username/password account with a custom avatar that requires an image upload
5. Make sure that for all registrations, the correct avatar appears in the ChatThreadList when the RegistrationNavigator dismisses

Reviewers: atul, ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8197