Clean up job concurrency calculation

- Mostly variable renaming and comments.
- A few bugs have been fixed.
- I *think* the max job count check is not working as intended, but we don't
fix it yet.

Bug: 111360323
Test: atest CtsJobSchedulerTestCases

Summary
-------
CtsJobSchedulerTestCases: Passed: 33, Failed: 5

5 tests failed
--------------
android.jobscheduler.cts.BatteryConstraintTest#testBatteryNotLowConstraintFails_withoutPower
android.jobscheduler.cts.ConnectivityConstraintTest#testConnectivityConstraintExecutes_metered
android.jobscheduler.cts.ConnectivityConstraintTest#testConnectivityConstraintExecutes_withMobile
android.jobscheduler.cts.ConnectivityConstraintTest#testUnmeteredConstraintFails_withMobile
android.jobscheduler.cts.TimingConstraintsTest#testJobParameters_unexpiredDeadline

(Unchanged from the ToT result)

Change-Id: I9f0fe8c18265652ca2b6217b3f6540aa7233b7c0