[backport#15930] rpcwallet: Make helper methods const on CWallet

Summary:
https://github.com/bitcoin/bitcoin/pull/15930/commits/fad13e925e197163a942f3f0d1ba2c95a2b65a56

Depends on D6444

Partial backport of Core PR15930

Test Plan:

ninja

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6445