sched: window-stats: use policy_mutex in sched_set_window()

Several configuration variable change will result in
reset_all_window_stats() being called. All of them, except
sched_set_window(), are serialized via policy_mutex. Take
policy_mutex in sched_set_window() as well to serialize use of
reset_all_window_stats() function

Change-Id: Iada7ff8ac85caa1517e2adcf6394c5b050e3968a
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>