ANDROID: uid_sys_stats: Replace tasklist lock with RCU in uid_cputime_show

Tasklist lock is acuquired in uid_cputime_show for updating the stats
for all tasks in the system. This can potentially disable preemption
for several milli seconds. Replace tasklist_lock with RCU read side
primitives.

Change-Id: Ife69cb577bfdceaae6eb21b9bda09a0fe687e140
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>