[Cashtab] Sort saved wallets alphabetically

Summary:
Wallets are currently sorted by order of creation. This is not ideal for Cashtab users with many wallets.

Sort by wallet name.

Test Plan: npm start, navigate to Settings screen, expand SavedWallets dropdown, confirm wallets are in alphabetical order

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15120