Reserve execution slots for FGS jobs.

Explicitly reserve execution slots for FGS apps so we don't force their
jobs to wait for truly background app jobs to finish first.

Also simplify the work count tracking code.

Bug: 181241779
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: I196f6dfc0666550e9dac4c443fda27394544ea3f