[Cashtab] [wallet mgmt improvements p9] Patch async function call bug in useEffect

Summary:
T2822

Depends on D12849

updateSavedWallets is an async function and should be called with await in an async useEffect

Test Plan:
npm start
go to settings page
change active wallet
Change it back

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12850