Fix job preemption logic

We try to find the lowest-priority running job for preemption. The code
was looking at the wrong piece of information for deciding which to
select, leading to preemption of inappropriate jobs.

Along the way, fix the mock-based JobSchedulerServiceTest, which had not
yet been updated to track AppStandInternal LocalService changes.

Bug: 137019975
Test: atest com.android.server.job.JobSchedulerServiceTest
Test: atest CtsJobSchedulerTestCases
Change-Id: I902a7626e4a14c288388544f8a6b263770805a7f