FirebaseRemoteConfigTest.java: fix flaky test: realtime_updatesBackoffMetadataWithProvidedRetryInterval (#7219)

Author: dconeybe

firebase-config/src/test/java/com/google/firebase/remoteconfig/FirebaseRemoteConfigTest.java

@@ -1577,7 +1577,7 @@ public void realtime_updatesBackoffMetadataWithProvidedRetryInterval() throws Ex
           long backoffDurationInMillis = expectedRetryIntervalInSeconds * 1000L;
           Date expectedBackoffEndTime = new Date(currentTime.getTime() + backoffDurationInMillis);
           return Math.abs(argument.getTime() - expectedBackoffEndTime.getTime())
-              <= TimeUnit.SECONDS.toSeconds(1);
+              <= TimeUnit.SECONDS.toMillis(1);
         };
 
     verify(sharedPrefsClient, times(1))