Add Standby Bucket and job duration to ScheduledJobStateChanged atom.

This fields enable us to slice job metrics by bucket and gain insight
to the average time spent executing jobs in each bucket. The metrics
we'll write for this data will help to tune thresholds for throttling
background behavior in Q.

BUG: 132227621
Test: make statsd_testdrive && statsd_testdrive 8

Change-Id: I1af94dc4a8e51f9741f48c817d1d2f99f164858d