[avalanche] Improve API for setContenderStatusForLocalWinners

Summary:
These changes make this function more testable and prepares it for
reconciling pollable contenders without re-calling getPollableContenders
unnecessarily.

Inspired by this feedback: https://reviews.bitcoinabc.org/D17607#inline-77172

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17660