[avalanche] Rename isAvalancheFinalized to isAvalancheFinalizedPreConsensus

Summary:
This method only checks for finalization via preconsensus, the actual name is confusing even if it belongs to the mempool.

There is no change in behavior.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18419