Fix the issue that background task doesn't work in Android.

I have confirmed that the worker task works better if the constraint `RequiresDeviceIdle` is not set to true.

Covid19Radar/Covid19Radar.Android/MainApplication.cs

@@ -24,5 +24,17 @@ public class MainApplication : Application
         public MainApplication(IntPtr handle, JniHandleOwnership transfer) : base(handle, transfer)
         {
         }
+
+        public override void OnCreate()
+        {
+            base.OnCreate();
+
+            Xamarin.ExposureNotifications.ExposureNotification.ConfigureBackgroundWorkRequest(TimeSpan.FromHours(6), b =>
+                b.SetConstraints(new AndroidX.Work.Constraints.Builder()
+                    .SetRequiresBatteryNotLow(true)
+                    .SetRequiredNetworkType(AndroidX.Work.NetworkType.Connected)
+                    .Build())
+                );
+        }
     }
 }