[ExposureNotification]Change ExistingPeriodicWorkPolicy

XPlat/ExposureNotification/source/Xamarin.ExposureNotification/ExposureNotification.android.cs

@@ -159,7 +159,7 @@ static Task PlatformScheduleFetch()
 			var workRequest = workRequestBuilder.Build();
 
 			workManager.EnqueueUniquePeriodicWork("exposurenotification",
-				ExistingPeriodicWorkPolicy.Replace,
+				ExistingPeriodicWorkPolicy.Keep,
 				workRequest);
 
 			return Task.CompletedTask;