[avalanche] Move block-vote decision into getAvalancheVoteForBlock.

Summary:
Abstract block-vote logic into its own function to prepare for adding other types of vote
logic.

Depends on D10685.

Test Plan: python test/functional/abc_p2p_avalanche_voting.py

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, deadalnix, Fabien

Subscribers: Fabien, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D10686