Init of OverlayManagerService on a parallel thread

Expensive parts of initialization (like idmap) are now done on a separate
init thread.

The service waits for init to complete in PHASE_SYSTEM_SERVICES_READY,
before affected apps can start.

Boot time saving: ~300ms on marlin.

Test: rebooted and verified that OverlayManagerService init is completed
      before any app process is started
Bug: 35949883

Change-Id: Ibdf0174c50cfa95fc8f1cb014ad0a03ff49f4d1f