Pass fImporting to ImportingNow helper class, Move ::fImporting to BlockManager

Summary:

It seems preferable to assign globals to a class (in this case BlockManager), than to leave them dangling. This should clarify scope for code-readers, as well as clarifying unit test behaviour.

This is a partial backport of core#25781
https://github.com/bitcoin/bitcoin/pull/25781/commits/fa442b137764e0b6c0d991ba641e90c3217be1bf
https://github.com/bitcoin/bitcoin/pull/25781/commits/fa9bd7be472f49b15f5f87711094095954322635

Depends on D16008

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16009