[avalanche] Encapsulate the activation functions

Summary:
This avoid accessing the flags directly, which are now private. This makes it possible to add more activation logic as well.
There is no change in behavior.

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18677