ANDROID: uid_cputime: add per-uid IO usage accounting

IO usages are accounted in foreground and background buckets.
For each uid, io usage is calculated in two steps.

delta = current total of all uid tasks - previus total
current bucket += delta

Bucket is determined by current uid stat. Userspace writes to
/proc/uid_procstat/set <uid> <stat> when uid stat is updated.

/proc/uid_io/stats shows IO usage in this format.
<uid> <foreground IO> <background IO>

Signed-off-by: Jin Qian <jinqian@google.com>
Bug: 34198239
Change-Id: I3369e59e063b1e5ee0dfe3804c711d93cb937c0c
Signed-off-by: Francisco Franco <franciscofranco.1990@gmail.com>