[native] Factor out loading spinner from SIWEPanel

Summary:
We want to use SIWEPanel in the new registration flow, but it currently renders a large spinner with flex: 1 while loading, which doesn't match the designs for the new registration flow.

To make SIWEPanel usable in both the old and new context, this diff factors out the spinner code into FullscreenSIWEPanel, making SIWEPanel more reusable.

Depends on D7840

Test Plan: Make sure the loading spinner still appears full-screen in the old experience

Reviewers: ginsu, atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7841