[Cashtab] OrderBook show prices in XEC by default

Summary:
This setting is more practical for working with OrderBooks. It is easier to distinguish price differences in different offers when prices are in XEC and not fiat.

Update tests to confirm new expected behavior.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17435