sched: Make wallclock more accurate

update_task_ravg() in context switch uses wallclock that is updated
before running put_prev_task() and pick_next_task(), both of which can
take some time. Its better to update wallclock after those routines,
leading to more accurate accounting.

Change-Id: I882b1f0e8eddd2cc17d42ca2ab8f7a2841b8d89a
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>