Change default value from true to false for preferential network service enabling

Test: atest DevicePolicyManagerTest#testSetGetPreferentialNetworkServiceEnabled;
      atest DevicePolicyManagerTest#testUpdateNetworkPreferenceOnStartUser;
      atest DevicePolicyManagerTest#testUpdateNetworkPreferenceOnStopUser;
      atest MixedManagedProfileOwnerTest#testSetPreferentialNetworkServiceStatusLogged;
      atest MixedManagedProfileOwnerTest#testSetGetPreferentialNetworkServiceStatus

Bug: 187771747
Change-Id: I3c7af4de7fb2c8ed284a27ada9092450e32aeb89