[ecash-herald] Update var name to better reflect purpose

Summary:
Depends on D14226

This amount used to be XEC. After app was upgraded to support fiat amounts, this amount is either fiat or XEC (if coingecko price API fails).

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14244