uid_cputime: Avoids double accounting of process stime, utime and cpu_power in

uid_cputime: Avoids double accounting of process stime, utime and cpu_power in
task exit.

This avoids the race where a particular process is terminating and we
read the show_uid_stats. At this time since the task_struct still exists
and we will account for the terminating process as one of the active
task, where as the stats would have been added in the task exit
callback.

Bug: 22064385
Change-Id: Id2ae04b33fcd230eda9683a41b6019d4dd8f5d85
Signed-off-by: Jin Qian <jinqian@google.com>
Signed-off-by: Ruchi Kandoi <kandoiruchi@google.com>
(cherry picked from commit ddd9aac59e0aea3d5a4df485173115e8ecb3e17d)