Reserve execution slot for important bg user jobs.

Explicitly reserve execution slots for FGS apps and EJs of a bg user
so they can be prioritized ahead of regular bg user jobs.

Bug: 171305774
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: I422773fdbdba00cbaccce68c4a6a38b4a4efb572