[backport#17304 5/18] Refactor: Move SetWalletFlag out of LegacyScriptPubKeyMan::UpgradeKeyMetadata

Summary:
This commit does not change behavior.

https://github.com/bitcoin/bitcoin/pull/17304/commits/4c5491f99c6b8ca779c48be63eb2ba18d98ee52a

Depends on D7151

Partial backport of Core PR17304

Test Plan:

ninja check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7152