sched: window-stats: Note legacy mode in fork() and exit()

In legacy mode, mark_task_starting() should avoid adding (new) task's
(initial) demand to rq->curr_runnable_sum and rq->prev_runnable_sum.
Similarly exit() should avoid removing (exiting) task's demand from
rq->curr_runnable_sum and rq->prev_runnable_sum (as those counters
don't include task's demand and partial_demand values in legacy mode).

Change-Id: I26820b1ac5885a9d681d363ec53d6866a2ea2e6f
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>