Avoid Environment.SpecialFolder.ApplicationData (#188)

* Don't use ApplicationData for config file on UNIX

When running on NET6 (VSMac installer), SpecialFolder.ApplicationData
returns nothing, resulting in a relative path being used for the config
file. So, just switch to use UserProfile, to which we append .config
for the well known, and expected, place for the config file.

Example testing with Mono's `csharp` command:

    $ csharp
    Mono C# Shell, type "help;" for help
    Enter statements below.
    csharp> Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData)                                                                                      
    "/Users/jopepper/.config"                                                                                                                                     
    csharp> Environment.GetFolderPath(Environment.SpecialFolder.UserProfile)                                                                                          
    "/Users/jopepper"     

Co-authored-by: Jonathan Peppers <jonathan.peppers@microsoft.com>

src/Xamarin.Android.Tools.AndroidSdk/Sdks/AndroidSdkUnix.cs

@@ -223,8 +223,7 @@ void AddToList (string path)
 			get {
 				var p   = AppDomain.CurrentDomain.GetData (GetUnixConfigDirOverrideName)?.ToString ();
 				if (string.IsNullOrEmpty (p)) {
-					p   = Environment.GetFolderPath (Environment.SpecialFolder.ApplicationData);
-					p   = Path.Combine (p, "xbuild");
+					p   = Path.Combine (Environment.GetFolderPath (Environment.SpecialFolder.UserProfile), ".config", "xbuild");
 				}
 				return Path.Combine (p, "monodroid-config.xml");
 			}