[Apps][Examples] Refactor sendXec demo to use updated API in ecash-coinselect v2.0.1

Summary: Update sendXec example to use the new API in ecash-coinselect v2.0.1.

Test Plan:
npm test
Fill in sender mnemonic and address and execute npm run sendXec <address> <amountInXec> and ensure it is broadcasted successfully via explorer.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D14571