Bug 457504 - Publish a job group's final status to IJobChangeListeners

Publish a job group's final status by extending the IJobChangeEvent
with a new getJobGroupResult() method, and when the last job in a
job group has executed, have the JobManager update the IJobChangeEvent
with the job group's final status.

Change-Id: I641f2670bab15d0a9af67e928e7b78a57718878f
Signed-off-by: Terry Parker <tparker@google.com>