Move JobScheduler off the main thread.

Move JobScheduler's handlers off of the main system thread onto a more
dedicated thread.

Bug: 268097442
Test: atest AppStandbyControllerTests
Test: atest CtsAlarmManagerTestCases
Test: atest CtsJobSchedulerTestCases
Test: atest frameworks/base/services/tests/mockingservicestests/src/com/android/server/alarm
Test: atest frameworks/base/services/tests/mockingservicestests/src/com/android/server/job
Test: atest frameworks/base/services/tests/mockingservicestests/src/com/android/server/tare
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/job
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/tare
Change-Id: I88ed0cf04f9e9e0f0e6dca7cfd96c66eeb811e3b