[Cashtab] Improve wallet select behavior on wallets screen

Summary:
Depends on D17557

Similar to how we improved wallet select behavior from the dropdown menu.

We already have a test for this screen to confirm user can activate different wallets. So, that it continues to pass confirms expected behavior. We do not test that "we expect it to take a little longer to load and have the UI locked until the new wallet loads."

Test Plan:
npm test

https://cashtab-local-dev.netlify.app/

Activate different wallets from the "Wallets" screen. See a spinner while the wallet loads.

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17558