Fix DatePicker dialog regression (#249)

KitKat doesn't use the cancel button, so it has to be accounted for.

Xamarin.Forms.Platform.Android/Renderers/DatePickerRenderer.cs

@@ -31,7 +31,9 @@ protected override void Dispose(bool disposing)
 				_disposed = true;
 				if (_dialog != null)
 				{
-					_dialog.CancelEvent -= OnCancelButtonClicked;
+					if (Forms.IsLollipopOrNewer)
+						_dialog.CancelEvent -= OnCancelButtonClicked;
+
 					_dialog.Hide();
 					_dialog.Dispose();
 					_dialog = null;
@@ -85,7 +87,10 @@ internal override void OnFocusChangeRequested(object sender, VisualElement.Focus
 				_dialog.Hide();
 				((IElementController)Element).SetValueFromRenderer(VisualElement.IsFocusedPropertyKey, false);
 				Control.ClearFocus();
-				_dialog.CancelEvent -= OnCancelButtonClicked;
+
+				if (Forms.IsLollipopOrNewer)
+					_dialog.CancelEvent -= OnCancelButtonClicked;
+
 				_dialog = null;
 			}
 		}
@@ -99,7 +104,9 @@ void CreateDatePickerDialog(int year, int month, int day)
 				((IElementController)view).SetValueFromRenderer(VisualElement.IsFocusedPropertyKey, false);
 				Control.ClearFocus();
 
-				_dialog.CancelEvent -= OnCancelButtonClicked;
+				if (Forms.IsLollipopOrNewer)
+					_dialog.CancelEvent -= OnCancelButtonClicked;
+
 				_dialog = null;
 			}, year, month, day);
 		}
@@ -128,7 +135,9 @@ void OnTextFieldClicked()
 			UpdateMinimumDate();
 			UpdateMaximumDate();
 
-			_dialog.CancelEvent += OnCancelButtonClicked;
+			if (Forms.IsLollipopOrNewer)
+				_dialog.CancelEvent += OnCancelButtonClicked;
+
 			_dialog.Show();
 		}