[Cashtab] Get change address from sending utxo

Summary: Change addresses were taken from wallet local storage. However, a more robust solution is to always get the change address from the spending utxo. Now that we are well past migrating to the 1899 deriv path, the wallet can go back to this method.

Test Plan:
npm test

npm start
send an xec transaction and confirm change address matches spending address
send an etoken transaction and confirm change address matches spending address

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9733