docs(Services): improve docs for local storage service

Signed-off-by: arafaysaleem <a.rafaysaleem@gmail.com>

lib/services/local_storage/prefs_base.dart

@@ -1,6 +1,6 @@
 import 'package:shared_preferences/shared_preferences.dart';
 
-///Internal class for shared preferences methods
+///Base class for shared preferences methods
 ///This class provides low level preferences methods
 class PrefsBase{
   ///Instance of shared preferences

lib/services/local_storage/prefs_service.dart

@@ -9,57 +9,67 @@ import '../../models/user_model.dart';
 //states
 import '../../providers/states/auth_state.dart';
 
+/// A service class for providing methods to store and retrieve data from
+/// shared preferences.
 class PrefsService {
-  final authTokenKey = "authToken";
-  final authStateKey = "authStateKey";
-  final authPasswordKey = "authPasswordKey";
-  final authUserKey = "authUserKey";
+
+  /// The name of auth token key
+  static const _authTokenKey = "authToken";
+
+  /// The name of auth state key
+  static const _authStateKey = "authStateKey";
+
+  /// The name of user password key
+  static const _authPasswordKey = "authPasswordKey";
+
+  /// The name of user model key
+  static const _authUserKey = "authUserKey";
 
   ///Instance of prefs class
   final _prefs = PrefsBase.instance;
 
   ///Returns logged in user password
   String getAuthPassword() {
-    return _prefs.get<String>(authPasswordKey) ?? '';
+    return _prefs.get<String>(_authPasswordKey) ?? '';
   }
 
   ///Returns last authentication status
   bool getAuthState() {
-    return _prefs.get<bool>(authStateKey) ?? false;
+    return _prefs.get<bool>(_authStateKey) ?? false;
   }
 
   ///Returns last authenticated user
   UserModel? getAuthUser() {
-    final user = _prefs.get<String>(authUserKey);
+    final user = _prefs.get<String>(_authUserKey);
     if(user == null) return null;
     return UserModel.fromJson(jsonDecode(user));
   }
 
   ///Returns last authentication token
   String getAuthToken() {
-    return _prefs.get<String>(authTokenKey) ?? '';
+    return _prefs.get<String>(_authTokenKey) ?? '';
   }
 
   ///Sets the authentication password to this value
   void setAuthPassword(String password) {
-    _prefs.set<String>(authPasswordKey, password);
+    _prefs.set<String>(_authPasswordKey, password);
   }
 
   ///Sets the authentication status to this value
   void setAuthState(AuthState authState) {
     if(authState is AUTHENTICATED) {
-      _prefs.set<bool>(authStateKey, true);
+      _prefs.set<bool>(_authStateKey, true);
     }
   }
 
   ///Sets the authenticated user to this value
   void setAuthUser(UserModel user) {
-    _prefs.set<String>(authUserKey, jsonEncode(user.toJson()));
+    _prefs.set<String>(_authUserKey, jsonEncode(user.toJson()));
   }
 
   ///Sets the authentication token to this value
   void setAuthToken(String token) {
-    _prefs.set<String>(authTokenKey, token);
+    _prefs.set<String>(_authTokenKey, token);
   }
 
   ///Resets the authentication