Make persisted job loading asynchronous.

Instead of loading all persisted jobs inside of JobSchedulerService's
constructor, start loading them asynchronously and make sure they're
fully loaded before we enter PHASE_SYSTEM_SERVICES_READY.

Bug: 259342534
Test: atest frameworks/base/services/tests/mockingservicestests/src/com/android/server/job
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/job
Test: atest CtsJobSchedulerTestCases
Change-Id: Ibccfb4de420c348492cf8240137a10e644d30e72