[Cashtab] Use live wallet.state tokens obj if present

Summary:
Now that Cashtab is using wallet.state instead of the legacy walletState field, make sure SendToken uses this value if we are dealing with a migrated wallet

Add unit tests similar to D9451 to ensure SendToken can handle migrated wallet, unmigrated wallet, and undefined wallet

Test Plan: npm start and confirm you can successfully send an SLPA transaction

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9467