sched: fix misalignment between requested and actual windows

When set_window_start() is first executed sched_clock() has not yet
stabilized. Refresh the sched_init_jiffy and sched_clock_at_init_jiffy
values until it is known that sched_clock has stabilized - this will
be the case by the time a client calls the sched_set_window() API.

Change-Id: Icd057707ff44c3b240e5e7e96891b23c95733daa
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>