Reserve execution slots for expedited jobs.

Make sure we reserve at least 3 slots for expedited jobs (in the ideal
case) so we can get them to run as quickly as possible.

Also fix how we assign slots when there are already running jobs.

Bug: 171305774
Test: atest CtsJobSchedulerTestCases
Test: atest FrameworksMockingServicesTests:JobSchedulerServiceTest
Test: atest FrameworksServicesTests:PrioritySchedulingTest
Test: atest FrameworksServicesTests:WorkCountTrackerTest
Test: atest FrameworksServicesTests:WorkTypeConfigTest
Change-Id: Ic08bc9d665c4c016f01ddce8f952710e72bd27f9