[Cashtab] Use available sk and pk rather than deriving from wif and ecc

Summary:
Now that sk and pk are calculated on cashtab startup and saved with the wallet object, we do not have to derive sk from wif and pk from sk

Had to update some more missed legacy types in mocks to implement.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17188