Fix flaky test: testDataMigration_differentFromFallback

Test main thread mocked settings object but use that on the handler
thread as well, which is not thread-safe.
Create a real object instead of mocking it instead.

Test: atest ConnectivityCoverageTests:android.net.connectivity.com.android.server.net.NetworkStatsServiceTest#testDataMigration_differentFromFallback \
      --rerun-until-failure 500
Fix: 289705810
Change-Id: Ibfb722818467fdf2b69a4b8fa87b4ac04a713573