src/main/java/io/reactivex/plugins/RxJavaPlugins.java
public static void setInitNewThreadSchedulerHandler(@Nullable Function<? super Callable<Scheduler>, ? extends Scheduler> handler) {
        if (lockdown) {
            throw new IllegalStateException("Plugins can't be changed anymore");
        }
        onInitNewThreadHandler = handler;
    }
public static void setInitNewThreadSchedulerHandler(@Nullable Function<? super Callable<Scheduler>, ? extends Scheduler> handler) {
        if (lockdown) {
            throw new IllegalStateException("Plugins can't be changed anymore");
        }
        onInitNewThreadHandler = handler;
    }
Sets the specific hook function.
@param handler the hook function to set, null allowed, but the function may not return null