Add DiskIo to Statsd

Adds diskio information to statsd. The puller queries the
proc/uid_io/stats file to get cumulative counts since boot.

Bug: b/116331466

Test: unit test
Test: manually verified that the puller and "cat /proc/uid_io/stats"
returned almost identical output when executed at the same time

Change-Id: Iac446f8dd879ab6bf859eed6e779cc16fdee6c5b