Refactored all providers to export singletons to avoid multiple instantiation of them

lib/src/resources/authService.dart

@@ -8,8 +8,8 @@ import '../models/summary_model.dart';
 import '../models/user_model.dart';
 
 class AuthService {
-  final _googleSignInProvider = GoogleSignInProvider();
+  final GoogleSignInProvider _googleSignInProvider = signInProvider;
-  final _firestoreProvider = FirestoreProvider();
+  final FirestoreProvider _firestoreProvider = firestoreProvider;
   final _user = BehaviorSubject<FirebaseUser>();
 
   Observable<FirebaseUser> get userStream => _user.stream;

lib/src/resources/firebase_storage_provider.dart

@@ -34,3 +34,5 @@ class FirebaseStorageProvider {
     return _storage.child(path).delete();
   }
 }
+
+final storageProvider = FirebaseStorageProvider();

lib/src/resources/firestore_provider.dart

@@ -295,3 +295,5 @@ class FirestoreProvider {
     return Observable(mappedStream);
   }
 }
+
+final firestoreProvider = FirestoreProvider();

lib/src/resources/google_sign_in_provider.dart

@@ -62,3 +62,5 @@ class GoogleSignInProvider {
     await _auth.signOut();
   }
 }
+
+final signInProvider = GoogleSignInProvider();

lib/src/resources/repository.dart

@@ -10,8 +10,8 @@ import '../models/task_model.dart';
 import '../models/summary_model.dart';
 
 class Repository {
-  final _storageProvider = FirebaseStorageProvider();
+  final FirebaseStorageProvider _storageProvider = storageProvider;
-  final _firestoreProvider = FirestoreProvider();
+  final FirestoreProvider _firestoreProvider = firestoreProvider;
 
   //--------------------------------CRUD----------------------------------------
   Future<void> updateUser(