sched: window-stats: Reset all window stats

Currently, few of the window statistics for tasks are not reset when
window size is changing. Fix this to completely reset all window
statistics for tasks and cpus. Move the reset code to a function,
which can be reused by a subsequent patch that resets same statistics
upon policy change.

Change-Id: Ic626260245b89007c4d70b9a07ebd577e217f283
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>