[ecash-herald] Remove hardcoded string from function

Summary:
T2972

Depends on D14082

Replace 6a with constant. Should also rename the constant, but another diff for that, used elsewhere.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D14083