SystemService post-completed user event: jobs

Use the new onUser completed event for JobSchedulerService.

This delays handling aspects of the user starting/unlock events
(which are important, but not urgent) until a little later.
In particular, it delays the intial job scheduling until a little after
the user has started, instead of immediately.

Test: manual
Bug: 197344658
Change-Id: I78ea7f7822ee466ce34dfc0b9f602fa1a4ceb7c3