[native] Refocus ConnectEthereum if registration fails due to expired nonce

Summary:
Prior to this diff stack, when registration fails due to an expired nonce we would dismiss the entire RegistrationNavigator.

This diff addresses ENG-4087 by making sure we refocus the ConnectEthereum screen when we get an expired nonce error.

I had to introduce RegistrationRouter here so that we could intelligently refocus ConnectEthereum, even if it's missing from the navigation stack.

Depends on D12018

Test Plan:
I tested this diff as follows:

1. Be in a multi-keyserver environment, testing SIWE with iOS simulator
2. Do a SIWE and then wait 2 minutes to let the nonce expire in the following screens:
   • FullscreenSIWEPanel for an account that doesn't exist yet when the new registration flow is disabled
   • FullscreenSIWEPanel for an account that doesn't exist yet when the new registration flow is enabled
   • FullscreenSIWEPanel for an account that does exist
   • New registration flow for an account that doesn't exist yet (RegistrationTerms)
   • New registration flow for an account that does exist (ExistingEthereumAccount)
3. Make sure there are no duplicate Alerts, that in all cases an Alert is shown, and that the "back" action activates when the user confirms the Alert

Reviewers: inka

Reviewed By: inka

Subscribers: tomek, varun

Differential Revision: https://phab.comm.dev/D12019