Expose job stop reasons.

Provide the various reasons a job could be stopped (via onStopJob) so
developers can react accordingly.

Bug: 171305774
Test: atest frameworks/base/services/tests/mockingservicestests/src/com/android/server/job
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/job
Test: atest CtsJobSchedulerTestCases
Change-Id: I5a19e74d3924de449cc5e4de27445e25cf8e25f5