Add "dumpsys diskstats" to get a dump of disk-free values.

Also includes a trivial test of I/O latency.  (Mostly useful to be invoked
from checkin to poll disk-free and latency issues.  Also moderately useful
to see in a bugreport.)

Also make "dumpsys netstat" a non-no-op, in case we want this data.

Change-Id: Ia93550a23ca6b35586c5d9217890ee0a5801aae1