More work on issue #26390151: Add new JobScheduler API...

...for monitoring content providers

- Improve media provider change reporting so that observers can
  avoid spurious reports of the top-level content directory changing.
- Fix a bug where collected content changes while a job was running
  were not being properly propagated to the next job.

Change-Id: I29e3c2960e6fec75b16ee3ee6588d47342bf8c75