Replace direct accesses to sched_active_thread and sched_active_pid with
the helper functions thread_getpid() and thread_get_active(). This serves
two purposes:
1. It makes accidental writes to those variable from outside core less likely.
2. Casting off the volatile qualifier is now well contained to those two
functions
Previously, sched_statistics_cb() was always called with two valid PIDs.
Now it is possible (when the idle thread is not used) that one of the
two might be KERNEL_PID_UNDEF, as the callback might be called when
descheduling the last thread, or scheduling the first.
