Make _activeProgressDialogs readonly

src/keepass2android/app/App.cs

@@ -466,7 +466,7 @@ namespace keepass2android
             }
         }
 
-		private HashSet<RealProgressDialog> _activeProgressDialogs = new HashSet<RealProgressDialog>();
+		private readonly HashSet<RealProgressDialog> _activeProgressDialogs = new HashSet<RealProgressDialog>();
 		// Whether the app is currently showing a dialog that requires user input, like a yesNoCancel dialog
 		private bool _isShowingUserInputDialog = false;