[avalanche] Use a constant for the default of -avalanchepreconsensus

Summary:
We are going to add more gating on this flag while the feature is being developped, so better use a constant to make it easier to maintain.

There is no change in behavior.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15270