[native] Introduce ExistingEthereumAccount screen

Summary:
See designs here. Actual login functionality and display of existing Ethereum account info will come in a later diff.

Depends on D7867

Test Plan:

1. I tested that if I attempted to connect an Ethereum account that already corresponded to an existing Comm account, I got sent to this new screen
2. I made sure the "Use a different wallet" button navigated back to the previous screen (ConnectEthereum)

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7875