[avalanche] Make the staking rewards activation chain dependent

Summary:
This makes it possible to default enable staking rewards on mainnet only. The feature is not useful on test chains and can always be enabled manually.

Depends on D14517.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK, bytesofman

Reviewed By: #bitcoin_abc, PiRK, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14604