[ecash-herald] Show the date of the activity in the msg header

Summary:
We are currently showing the "next" date since we build the msg at UTC midnight. But the activity is from the previous day.

Make sure we show the previous day's date.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17731