sched: window-stats: adjust RQ curr, prev sums on task migration

Adjust cpu's busy time in its recent and previous window upon task
migration. This would enable scheduler to provide better inputs to
cpufreq governor on a cpu's busy time in a given window.

Change-Id: Idec2ca459382e9f46d882da3af53148412d631c6
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>