ANDROID: uid_sys_stats: check previous uid_entry before call find_or_register_uid

ANDROID: uid_sys_stats: check previous uid_entry before call find_or_register_uid

Theads in a process are stored in list struct task_struct->thread_group,
so it will be visited continiously in below loop:
do_each_thread(temp, task) {
    ...
} while_each_thread(temp, task);

I add some log in the loop, we can see below information:
[   65.033561] uid 1000, uid_entry ffffffc0f2761600
[   65.033567] uid 1000, uid_entry ffffffc0f2761600
[   65.033574] uid 1000, uid_entry ffffffc0f2761600
[   65.033581] uid 1000, uid_entry ffffffc0f2761600
[   65.033588] uid 1000, uid_entry ffffffc0f2761600
[   65.033595] uid 1000, uid_entry ffffffc0f2761600
[   65.033602] uid 1000, uid_entry ffffffc0f2761600
[   65.033609] uid 1000, uid_entry ffffffc0f2761600
[   65.033615] uid 1000, uid_entry ffffffc0f2761600
[   65.033622] uid 1000, uid_entry ffffffc0f2761600
[   65.033629] uid 1000, uid_entry ffffffc0f2761600
[   65.033637] uid 1000, uid_entry ffffffc0f2761600
[   65.033644] uid 1000, uid_entry ffffffc0f2761600
[   65.033651] uid 1000, uid_entry ffffffc0f2761600
[   65.033658] uid 1000, uid_entry ffffffc0f2761600
[   65.033665] uid 1000, uid_entry ffffffc0f2761600
[   65.033672] uid 1000, uid_entry ffffffc0f2761600
[   65.033680] uid 1000, uid_entry ffffffc0f2761600
[   65.033687] uid 1000, uid_entry ffffffc0f2761600
[   65.033694] uid 1000, uid_entry ffffffc0f2761600
[   65.033701] uid 1000, uid_entry ffffffc0f2761600
[   65.033708] uid 1000, uid_entry ffffffc0f2761600
[   65.033715] uid 1000, uid_entry ffffffc0f2761600
[   65.033722] uid 1000, uid_entry ffffffc0f2761600
[   65.033729] uid 1000, uid_entry ffffffc0f2761600
[   65.033736] uid 1000, uid_entry ffffffc0f2761600
[   65.033743] uid 1000, uid_entry ffffffc0f2761600
[   65.033750] uid 1000, uid_entry ffffffc0f2761600
[   65.033757] uid 1000, uid_entry ffffffc0f2761600
[   65.033763] uid 1000, uid_entry ffffffc0f2761600
[   65.033770] uid 1000, uid_entry ffffffc0f2761600
[   65.033777] uid 1000, uid_entry ffffffc0f2761600
[   65.033784] uid 1000, uid_entry ffffffc0f2761600
[   65.033791] uid 1000, uid_entry ffffffc0f2761600
[   65.033798] uid 1000, uid_entry ffffffc0f2761600

So we can check the previous uid_entry before calling find_or_register_uid
to save time.

Change-Id: I05ec1a1405a80c0a620cb4b4b2f6483dbfde7829
Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>