REORG: sched: move idle time calculation from time.h to task.h

REORG: sched: move idle time calculation from time.h to task.h

time.h is a horrible place to put activity calculation, it's a
historical mistake because the functions were there. We already have
most of the parts in sched.{c,h} and these ones make an exception in
the middle, forcing time.h to include some thread stuff and to access
the before/after_poll and idle_pct values.

Let's move these 3 functions to task.h with the other ones. They were
prefixed with "sched_" instead of the historical "tv_" which already
made no sense anymore.