Fix concurrency limits.

The concurrency manager was accidentally limiting background apps to one
job at a time instead of correctly letting them run multiple jobs and
limiting them when they hit the package concurrency limit.

Bug: 204924801
Test: atest com.android.server.job.JobConcurrencyManagerTest
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/job
Test: atest frameworks/base/services/tests/mockingservicestests/src/com/android/server/job
Change-Id: I51d9c41ce0c47fc1f634f3df8753730d7d3c641a