[avalanche] Add a facility to check if a peer is preferred for DL

Summary:
This wraps up the access to the state's fPreferredDownload flag to
prevent code duplication.

There is no change in behavior.

Depends on D9582.

Part of T1611.

Test Plan:

ninja all check-all
./test/functional/test_runner.py p2p_inv_download

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1611

Differential Revision: https://reviews.bitcoinabc.org/D9625