[net processing] Move nPreferredDownload into PeerManagerImpl

Summary:
Note that UpdatePreferredDownload was inlined in D14747 (this would be the next commit in this PR)

This is a partial backport of core#24543
https://github.com/bitcoin/bitcoin/pull/24543/commits/490c08f96a34ed436c3d2cf7b9a3ed72694b6147

Depends on D14862

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14863