Simplify running job checking.

Now that we have a list of the currently running jobs, we no longer need
to iterate through all JobServiceContexts to know if a job is running.
Clean up the code by using the set of running jobs instead of iterating.

Bug: 141645789
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/job
Test: atest frameworks/base/services/tests/mockingservicestests/src/com/android/server/job
Test: atest CtsJobSchedulerTestCases
Change-Id: Icfdf420fa14496f0fd93f6aa6e67aaa229372985