[Cashtab] Bugfix in haveUtxosChanged

Summary:
The haveUtxosChanged function always returns true after it was switched to compare utxo from API against previously saved wallet.state utxo set. This means that hydrateUtxos is called every 10s, degrading app performance.

This diff returns to the original way of comparing utxo set.

Additionally, the diff simplifies the function by removing redundant and confusing if/else statements.

Test Plan: npm test

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9516