proc: uid: Adds accounting for the cputimes per uid

Adds proc files /proc/uid_cputime/show_uid_stat and
/proc/uid_cputime/remove_uid_range.

show_uid_stat lists the total utime and stime for the active as well as
terminated processes for each of the uids.

Writing a range of uids to remove_uid_range will delete the accounting
for all the uids within that range.

Change-Id: Ibaf562c66fef82c3a4d793c67e52e100d5f803a4
Signed-off-by: Jin Qian <jinqian@google.com>
Git-commit: 3aaf46626af5875372d625644759c570a1515c3f
Git-repo: https://android.googlesource.com/kernel/common/


[nabrah@codeaurora.org: Resolved trivial merge conflicts]
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>