Split SetWalletFlags into Add/LoadWalletFlags

Summary:
Remove memonly bool and follow typical Add and Load pattern used
everywhere else.

This is a backport of core#19046 [3/3]
https://github.com/bitcoin/bitcoin/pull/19046/commits/3a9aba21a49a6d80bd187940d5e26893937b6832

Depends on D9631

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9632