[native][landing] Fix up mobile SIWE flow after Wagmi and Rainbowkit updates

Summary:
Some changes here:

1. The main Rainbowkit modal doesn't reappear anymore when the WalletConnect sub-modal is dismissed. As a result, I've made some changes in landing/siwe.react.js which cause { type: 'siwe_closed' } to be passed to native when BOTH modals are not visible and a signer does not appear within 500ms.
2. The 500ms timeout mentioned above was previously 250ms. I found it necessary to increase during testing.
3. I've updated SIWEPanel to avoid resizing the WebView back to Rainbowkit size when the WalletConnect modal is dismissed. There's no point to doing this, since the Rainbowkit content doesn't display anyways, and the changes described above cause the WebView to be immediately closed anyways.

Depends on D11490

Test Plan: Testing closing main modal, closing WalletConnect modal, and WalletConnect modal workflow end-to-end in an iOS simulator

Reviewers: atul, ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11491