[MS62.1] Start NetworkStatsService from the module

NetworkStatsService is going to be moved into Connectivity mainlne
module and it will be not visible to SystemServiceResistry after
migration done. Thus:
1. Register service in NetworkStatsServiceInitialalizer, a separate
   initializer is needed because the service needs to be created
   before NetworkPolicyManagerService's initialization.
2. Call SystemReady at PHASE_ACTIVITY_MANAGER_READY phase where
   it needs to be ready before StatsPullAtomService pulls at
   PHASE_THIRD_PARTY_APPS_CAN_START.
3. Register manager in ConnectivityFrameworkInitializerTiramisu
   instead of SystemServiceRegistery.

Test: atest FrameworksNetTests CtsNetTestCases NetworkUsageStatsTest
Bug: 204830222
  (cherry-picked from ag/16688794)
Change-Id: I95020d816ed613ce3480a0720a5b71dcdc7fab72
Merged-In: I95020d816ed613ce3480a0720a5b71dcdc7fab72