Simple UseSeconds fix

Author: VisualMelon

samples/Sandbox/MainWindow.axaml

@@ -1,4 +1,9 @@
 <Window xmlns="https://github.com/avaloniaui"
         xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
         x:Class="Sandbox.MainWindow">
+  <StackPanel>
+    <TimePicker x:Name="Picker" />
+    <TextBlock Text="{Binding #Picker.SelectedTime}" />
+    <CheckBox IsChecked="{Binding #Picker.UseSeconds}" />
+  </StackPanel>
 </Window>

src/Avalonia.Controls/DateTimePickers/TimePickerPresenter.cs

@@ -241,7 +241,7 @@ protected override void OnConfirmed()
                 hr = per == 1 ? (hr == 12) ? 12 : hr + 12 : per == 0 && hr == 12 ? 0 : hr;
             }
 
-            SetCurrentValue(TimeProperty, new TimeSpan(hr, min, sec));
+            SetCurrentValue(TimeProperty, new TimeSpan(hr, min, UseSeconds ? sec : 0));
 
             base.OnConfirmed();
         }