[Cashtab] After importing a wallet, automatically close the import wallet modal

Summary:
T3531

Came across this using the app. The import wallet modal should close after we successfully import a new wallet.

Update behavior and tests to confirm behavior.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15970