2-phase initialization of NetworkPolicyManagerService

Split systemReady() into 2 callbacks:
1) networkScoreAndNetworkManagementServiceReady - called when required
    services are ready. NetworkPolicyManagerService starts its initialization
    on the worker thread
2) systemReady - wait for phase (1) to finish so that the service is fully
   initialized

NetworkPolicyManagerService is now prepared in parallel with
ConnectivityService and NetworkStatsService

Test: device boots without errors
Test: HostsideRestrictBackgroundNetworkTests and NetworkPolicyManagerServiceTest pass
Bug: 32315581
Change-Id: Ic9755ed096900247fa0bbb43f396d8bcb97ae6db