Add missing virtual keywords

Xamarin.Forms.Platform.Android/Cells/EntryCellRenderer.cs

@@ -58,7 +58,7 @@ namespace Xamarin.Forms.Platform.Android
 				UpdateHeight();
 		}
 
-		protected NumberKeyListener GetDigitsKeyListener(InputTypes inputTypes)
+		protected virtual NumberKeyListener GetDigitsKeyListener(InputTypes inputTypes)
 		{
 			// Override this in a custom renderer to use a different NumberKeyListener 
 			// or to filter out input types you don't want to allow 

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

@@ -87,7 +87,7 @@ namespace Xamarin.Forms.Platform.Android
 			base.OnElementPropertyChanged(sender, e);
 		}
 
-		protected NumberKeyListener GetDigitsKeyListener(InputTypes inputTypes)
+		protected virtual NumberKeyListener GetDigitsKeyListener(InputTypes inputTypes)
 		{
 			// Override this in a custom renderer to use a different NumberKeyListener 
 			// or to filter out input types you don't want to allow