[backport#17304 18/18] Refactor: Move nTimeFirstKey accesses out of CWallet

Summary:
This commit does not change behavior.

https://github.com/bitcoin/bitcoin/pull/17304/commits/152b0a00d8e681dd098f6b548447b82ab54ebe3c

Depends on D7164

Concludes backport of Core PR17304

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7165