[e.cash] Use avalanche price api for navbar

Summary: Switching to use the avalanche.cash XEC price API in the navbar price display for better reliabilty

Test Plan: preivew the site and check the price still populates as expected

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D16334