[native] Set incognito for SIWE WebView

Summary:
You can get stuck halfway through the SIWE flow if RainbowKit in JS thinks you're authenticated, but your Rainbow app doesn't.

By setting incognito={true}, we reset the state of the WebView on every open, so that we can start the flow from scratch.

Depends on D6035

Test Plan: Get stuck halfway through SIWE flow, then apply this diff and see myself automatically unstuck

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6036