[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