Add and implement JobScheduler content observer APIs.

Allows one to schedule jobs to run when content URIs
change, and find out what changed when the job executes.

This required adding a new API to StateController to
tell it when we are about to start executing a job, so
we can transfer the currently collected changes out of
its internal state to fill it in to the JobParameters.

Also some additional dumpsys debug output to help
understand what is going on in the job scheduler.

Change-Id: I91f51b226ff4add7a271a8333beffa5e86c7bf18