[mtouch] Fix [InternalsVisibleTo] attribute parsing. Fixes #4895 (#4921)

The input is a "user" string, i.e. not some well formatted metadata.
Spec [1] mention it's comma separated, but spaces are optional.

[1] https://docs.microsoft.com/en-us/dotnet/framework/reflection-and-codedom/specifying-fully-qualified-type-names#specifying-assembly-names

Fixes #4895

src/AssemblyInfo.cs.in

@@ -16,4 +16,4 @@ using System.Runtime.CompilerServices;
 // [assembly: AssemblyCopyright ("Copyright 2011-2014 Xamarin Inc.")]
 [assembly: AssemblyCompany ("Xamarin Inc.")]
 
-[assembly: InternalsVisibleTo ("System.Net.Http, PublicKey=002400000480000094000000060200000024000052534131000400000100010007d1fa57c4aed9f0a32e84aa0faefd0de9e8fd6aec8f87fb03766c834c99921eb23be79ad9d5dcc1dd9ad236132102900b723cf980957fc4e177108fc607774f29e8320e92ea05ece4e821c0a5efe8f1645c4c0c93c1ab99285d622caa652c1dfad63d745d6f2de5f17e5eaf0fc4963d261c8a12436518206dc093344d5ad293")]
+[assembly: InternalsVisibleTo ("System.Net.Http,PublicKey=002400000480000094000000060200000024000052534131000400000100010007d1fa57c4aed9f0a32e84aa0faefd0de9e8fd6aec8f87fb03766c834c99921eb23be79ad9d5dcc1dd9ad236132102900b723cf980957fc4e177108fc607774f29e8320e92ea05ece4e821c0a5efe8f1645c4c0c93c1ab99285d622caa652c1dfad63d745d6f2de5f17e5eaf0fc4963d261c8a12436518206dc093344d5ad293")]

tools/linker/MonoTouch.Tuner/MonoTouchSweepStep.cs

@@ -35,9 +35,9 @@ void SweepAttributes (IList<CustomAttribute> attributes)
 				// validating the public key and the public key token would be time consuming
 				// worse case (no match) is that we keep the attribute while it's not needed
 				var fqn = (ca.ConstructorArguments [0].Value as string);
-				int pk = fqn.IndexOf (", PublicKey=", StringComparison.OrdinalIgnoreCase);
-				if (pk != -1)
-					fqn = fqn.Substring (0, pk);
+				int comma = fqn.IndexOf (',');
+				if (comma != -1)
+					fqn = fqn.Substring (0, comma);
 
 				bool need_ivt = false;
 				foreach (var assembly in Context.GetAssemblies ()) {