[Cashtab] Move settings to new cashtabState

Summary:
T3445

Add settings to the new cashtabState key with its standardized loading and setting methods.

Implement throughout the app. Add a new integration test confirming that toggling a setting on Config screen changes behavior on SendXec screen.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15562