[Cashtab] Show fiat amount of incoming XEC txs

Summary:
T1725

Show the fiat amount of incoming XEC transactions on the notification in the user's selected local currency.

Test Plan: npm start, send yourself some XEC, check out the notification and make sure it looks okay and is calculating the right price.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9908