Fix flaky background fetch UT

The test asserted with a very strict timeline that `stopWork()` would
guarantee that no `runBackgroundFetch()` method would still be running,
but the addition of the logger introduced a few milliseconds of delay.

Test: atest com.android.adservices.service.customaudience.BackgroundFetchWorkerTest#testStopWorkGracefullyStopsBackgroundFetch --iterations 100
Bug: 260211148
Change-Id: I32c1ccbf36cbc547ddef2d038cfc5c6d1b057e70