sched: prevent race between disable window statistics and task grouping

Change of colocation group requires to finish CPU busy time accounting
prior to its operation by calling update_task_ravg().  However when
window statistics accounting is disabled, update_task_ravg() acts as
nop and results in incorrect CPU time accounting.

Disallow colocation group change while window statistics accounting is
disabled in order to prevent race between reset_all_window_stats() and
colocation grouping functions.

Change-Id: I6dfa20b8d8b0ae7ccc94119bf9cf14c5e11a1cf7
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>