[native] Don't refetch nonce after SIWE is complete

Summary: This is an alternative to D11771, which has been reverted because it caused ENG-7969.

Test Plan:

1. Tested main SIWE workflow
2. Tested backup secret generation via SIWE
3. Tested what happens when we get an expired nonce in the main SIWE workflow
4. Read through the code carefully (all three uses of SIWEPanel) to make sure that in all cases, once onClosing or onClosed is triggered, the SIWEPanel will be unmounted

I tested both SIWE and backup generation, as well as some error cases such as expired nonce. I also read through the code carefully to make

Reviewers: tomek, marcin

Reviewed By: marcin

Differential Revision: https://phab.comm.dev/D11794