Defer readSessionsLocked until system ready

readSessionsLocked depend on DPMS, so defer it until all system
service is started

Fix: 34663223

Test: Manual
      1. Try to install an app and reboot before it is installed
         (so the session is persisted in a file)
      2. Observe that PackageInstallerService
         read the persisted session in onSystemReady and 
         able to get the DPM instance.
      3. Observe play can read the persisted session

Change-Id: Ia14ed09db08fcf59189fa29293f0ac5066650942