Fix NamedSize in Previewer (#5496)

In the previewer, ConvertTextAppearanceToSize() returns -0.5 for all values. Return default value if we get a size of 0 or less.

Xamarin.Forms.Platform.Android/Forms.cs

@@ -549,9 +549,9 @@ namespace Xamarin.Forms
 			{
 				double myValue;
 
-				if (TryGetTextAppearance(themeDefault, out myValue))
+				if (TryGetTextAppearance(themeDefault, out myValue) && myValue > 0)
 					return myValue;
-				if (TryGetTextAppearance(deviceDefault, out myValue))
+				if (TryGetTextAppearance(deviceDefault, out myValue) && myValue > 0)
 					return myValue;
 				return defaultValue;
 			}