Init PersistentDatablockService on the worker thread

Other services do not depend on PersistentDatabalockService in the early
stages of system server initialization.
Initialization is now done on the FgThread. The service also ensures that
init is complete before PHASE_ACTIVITY_MANAGER_READY.

Change-Id: I3b9a1994e37c1e995ed04257f271c425d48c2e7c
Test: Manual, device boots, service is initialized and published
Bug: 33199244