[Xamarin.Android.Build.Tasks] Use full path to create designer output directory. (#1542)

Fixes: #1541

We missed one area in the changes where we worked around the
WorkingDirectory issues (45efffe): we were not using the full path
to create the directory where the java `R.java` files are output.
As a result we ended up with the following error:

	Error        Access to the path 'obj\Debug\monoandroid81\android\src' is denied.
	   at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
	   at System.IO.Directory.InternalCreateDirectory(String fullPath, String path, Object dirSecurityObj, Boolean checkHost)
	   at System.IO.Directory.InternalCreateDirectoryHelper(String path, Boolean checkHost)
	   at System.IO.Directory.CreateDirectory(String path)
	   at Xamarin.Android.Tasks.Aapt.GenerateCommandLineCommands(String ManifestFile, String currentAbi, String currentResourceOutputFile)
	   at Xamarin.Android.Tasks.Aapt.ProcessManifest(ITaskItem manifestFile)

Once again this is due to the working directory changing in the
thread that we are using.

So the solution is to use the full path to create the folder. In
theory because we use the `WorkingDirectory` on the process we should
NOT need to pass the full path as the command line argument. While
that might be a safer option we might well run into issues were we
exceed the max path or max command line length on Windows.

Author: dellis1972

src/Xamarin.Android.Build.Tasks/Tasks/Aapt.cs

@@ -330,7 +330,8 @@ protected string GenerateCommandLineCommands (string ManifestFile, string curren
 			manifest.Save (manifestFile);
 
 			cmd.AppendSwitchIfNotNull ("-M ", manifestFile);
-			Directory.CreateDirectory (JavaDesignerOutputDirectory);
+			var designerDirectory = Path.IsPathRooted (JavaDesignerOutputDirectory) ? JavaDesignerOutputDirectory : Path.Combine (WorkingDirectory, JavaDesignerOutputDirectory);
+			Directory.CreateDirectory (designerDirectory);
 			cmd.AppendSwitchIfNotNull ("-J ", JavaDesignerOutputDirectory);
 
 			if (PackageName != null)