sched: window-stats: Fix accounting bug in legacy mode

TASK_UPDATE event currently does not result in increment of
rq->curr_runnable_sum in legacy mode, which is wrong. As a result,
cpu busy time reported under legacy mode could be incorrect.

Change-Id: Ifa76c735a0ead23062c1a64faf97e7b801b66bf9
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>