sched: window-stats: Account idle time as busy time

Provide a knob to consider idle time as busy time, when cpu becomes
idle as a result of io_schedule() call. This will let governor
parameter 'io_is_busy' to be appropriately honored.

Change-Id: Id9fb4fe448e8e4909696aa8a3be5a165ad7529d3
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>