This removes a few callsites for the global g_avalanche. It makes sense to assume that a StakingRewardPolicy should be used only when the processor is initialized, passing a reference instead of invoking a pointer makes that a hard rule.
Note that the is IsStakingRewardsActivated function is always called after we already checked for g_avalanche in non-test code, so there is no change in behavior by removing that check. And this allows removing some setup code in stakingrewards_tests.cpp
Depends on D16259