[avalanche] Clarify separation of responsibilities for setting local winner contender statuses

Summary:
Move the peerManager logic into peerManager. This eliminates the public getPollableContenders
function and enables further cleanup related to contender finalization.

Test Plan:

ninja && ninja check && ./test/functional/test_runner.py abc_p2p_avalanche_contender_voting

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17881