Atom: ScheduledJobStateChanged

Rename the possible states in ScheduleJobStateChanged
from ON and OFF to SCHEDULED, STARTED, and FINISHED.
Added the log line in JobSchedulerService for SCHEDULE.
STARTED corresponds to what previously was the ON event.
FINISHED corresponds to what previously was the OFF event.

Test: accompanying CTS test
Change-Id: I779dd58092e8fdee58561b8e8e74be5404f3ed93