Create standby bucket changed atom.

We'll use the data from the atom events to further optimize our quotas.

Bug: 135417506
Bug: 143495340
Bug: 149869487
Test: Use statsd_testdrive to test WakeupAlarmOccurred and
AppStandbyBucketChanged are both logged properly

Change-Id: Id05304175dea804d83f6056bf4da2e049496d87d