upload new photos in background with a service (#382)

* properly done background backup service

* new concurrency/locking management with heartbeat

fix communication erros with Kotlin plugin on start/stop service methods

better error handling for BackgroundService public methods

Add default notification message when service is running

* configurable WiFi & charging requirement for service

* use translations in background service

mobile/lib/constants/locales.dart

@@ -0,0 +1,17 @@
+import 'dart:ui';
+
+const List<Locale> locales = [
+  // Default locale
+  Locale('en', 'US'),
+  // Additional locales
+  Locale('da', 'DK'),
+  Locale('de', 'DE'),
+  Locale('es', 'ES'),
+  Locale('fi', 'FI'),
+  Locale('fr', 'FR'),
+  Locale('it', 'IT'),
+  Locale('ja', 'JP'),
+  Locale('pl', 'PL')
+];
+
+const String translationsPath = 'assets/i18n';