Address various JobScheduler API feedback.

- New sample code.
- Fix/improve some docs.
- Hide JobWorkItem Parcl constructor.

Also:

- Add new JobWorkItem API to get the number of times it has been
delivered.
- Do a bit more optimization of checking if a job is ready.

Bug: 37534393  API Review: JobInfo.Builder
Bug: 37544057  API Review: JobServiceEngine
Bug: 37544153  API Review: JobWorkItem

Test: bit CtsJobSchedulerTestCases:*
Change-Id: I66891a038fba752f45dcaed43e615fa9209b71fc