[cashtab] Consistently use appConfig.ticker instead of hardcoded 'XEC'

Summary: This is so this can work on testnet.

Test Plan:

./contrib/teamcity/build-configurations.py cashtab-tests

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D18378