Migrate NetworkStatsServiceTest from when().thenReturn() to doReturn().when()

This will address flakiness issues arising from the use of
when() in multithread context.

Fix: 243658074
Test: atest com.android.server.net.NetworkStatsServiceTest#testDataMigration_differentFromFallback \
      --rerun-until-failure 1000
Change-Id: I047050ef14cb60658098ff7803c6773907a92919