[Cashtab] Patch bug in building token change outputs

Summary:
Lingering issue here where users may create a token tx that sends changes to another wallet they control that is not the active wallet.

This is because the constant we are using to manage token change is being mutated by sendXec. So a user sending multiple txs while changing wallets will run into this.

The "right way" to fix this is to have a wallet class that handles utxo mgmt and sends. Working on this. But should patch this now.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17545