[Cashtab] Reduce tx autoclose time from 5s to 2s

Summary: In regular use, I find myself closing txs to get to the UI behind them more often than I find myself glad that the notification is "still there" 5s later.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17591