[class-parse] Use InvariantCulture to format floating-point values (dotnet#131)

Fixes: dotnet#129

The System.Double/Single ["Round-trip Format Specifier"][0] varies by
culture, so cultures which use `,` as the decimal separator character
use `,` within e.g. `42.0.ToString("r")`.

For example, the Java declaration:

	class E {
		public static final double V = 42.5;
	}

on french systems would result in `class-parse` emitting:

	<field name="V" value="42,5" .../>

which, when run through `generator`, emits:

	public const double V = (double) 42,5;

...which promptly fails with a CS1001. Oops.

Fix our `double` and `float` serialization in
`XmlClassDeclarationBuilder` so that `CultureInfo.InvariantCulture` is
used, which ensures that `.` is the decimal separator character,
ensuring that `class-parse` and `generator` output is as expected.

[0]: https://msdn.microsoft.com/en-us/library/dwhawy9k(v=vs.110).aspx#RFormatString

Author: jonpryor

src/Xamarin.Android.Tools.Bytecode/XmlClassDeclarationBuilder.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Globalization;
 using System.Linq;
 using System.Xml.Linq;
 using System.Text;
@@ -443,9 +444,19 @@ static XAttribute GetValue (FieldInfo field)
 				else if (Double.IsPositiveInfinity (doubleItem.Value))
 					value = "(1.0 / 0.0)";
 				else
-					value = doubleItem.Value.ToString ("R");
+					value = doubleItem.Value.ToString ("R", CultureInfo.InvariantCulture);
+				break;
+			case ConstantPoolItemType.Float:
+				var floatItem = (ConstantPoolFloatItem) constant;
+				if (Double.IsNaN (floatItem.Value))
+					value = "(0.0f / 0.0f)";
+				else if (Double.IsNegativeInfinity (floatItem.Value))
+					value = "(-1.0f / 0.0f)";
+				else if (Double.IsPositiveInfinity (floatItem.Value))
+					value = "(1.0f / 0.0f)";
+				else
+					value = floatItem.Value.ToString ("R", CultureInfo.InvariantCulture);
 				break;
-			case ConstantPoolItemType.Float:    value = ((ConstantPoolFloatItem)constant).Value.ToString ("R");    break;
 			case ConstantPoolItemType.Long:     value = ((ConstantPoolLongItem) constant).Value.ToString ();    break;
 			case ConstantPoolItemType.Integer:
 				if (field.Descriptor == "Z")