Updates statsd atoms.proto with small changes.

Renames the apphook atom to be more descriptive. Renames time
suffixes to match more clear convention (ms and msec are ambiguous).

Adds a field to CPU active time that's required for the metrics
to be usable.

Test: Test that statsd can still build.
Change-Id: I94866510738db994e8d757260f30e599ba995dbd