[native] Make SIWEPanel more generic

Summary: This way we can use it from the new registration flow as well.

Test Plan:

1. Read the code and make sure that the only valid value of nextMode after siwe is prompt
2. Flow
3. Make sure the old SIWE flow still works correctly, and that all of the ways to close it (swipe away SIWEPanel, hit back button, and hit close button) all still work and animate correctly

Reviewers: ginsu, atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7840