Fix bug and migrate test.

Migrate WorkCountTrackerTest to specify WorkType limits explicitly. Also
fix bug in how we assign slots to pending & already running jobs.

Bug: 141645789
Bug: 171305774
Bug: 178119369
Test: atest CtsJobSchedulerTestCases
Test: atest FrameworksMockingServicesTests:JobSchedulerServiceTest
Test: atest FrameworksServicesTests:PrioritySchedulingTest
Test: atest FrameworksServicesTests:WorkCountTrackerTest
Test: atest FrameworksServicesTests:WorkTypeConfigTest
Change-Id: I741e071391819ad3575ef981e8feb287093aa387