[Xamarin.Android.Build.Utilities] Xamarin can't find NDK 12b because ndk-stack.exe is now ndk-stack.cmd

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=42566

The problem was that we did not UPDATE the ndk-stack.exe property
to be ndk-stack.cmd once we had verified that the ndk was valid.

This commit adds support for changing the ndk-stack extension to
one of the "known" extentions on Windows if it exists.

Author: dellis1972

src/Xamarin.Android.Build.Utilities/Sdks/AndroidSdkBase.cs

@@ -136,16 +136,24 @@ protected IEnumerable<string> FindExecutableInPath (string executable)
 
 		protected IEnumerable<string> FindExecutableInDirectory(string executable, string dir)
 		{			
-			var pathExt = Environment.GetEnvironmentVariable ("PATHEXT");
-			var pathExts = pathExt?.Split (new char [] { Path.PathSeparator }, StringSplitOptions.RemoveEmptyEntries);
-			
 			if (File.Exists (Path.Combine (dir, (executable))))
 				yield return dir;
+			
+			foreach (var exe in Executables (executable))
+				if (File.Exists (Path.Combine (dir, exe)))
+					yield return dir;
+		}
+
+		protected IEnumerable<string> Executables (string executable)
+		{
+			var pathExt = Environment.GetEnvironmentVariable ("PATHEXT");
+			var pathExts = pathExt?.Split (new char [] { Path.PathSeparator }, StringSplitOptions.RemoveEmptyEntries);
+
 			if (pathExts == null)
 				yield break;
+
 			foreach (var ext in pathExts)
-				if (File.Exists (Path.Combine (dir, Path.ChangeExtension (executable, ext))))
-					yield return dir;
+				yield return Path.ChangeExtension (executable, ext);
 		}
 
 		protected string NullIfEmpty (string s)

src/Xamarin.Android.Build.Utilities/Sdks/AndroidSdkWindows.cs

@@ -15,14 +15,16 @@ class AndroidSdkWindows : AndroidSdkBase
 		const string XAMARIN_ANDROID_INSTALLER_PATH = @"SOFTWARE\Xamarin\MonoAndroid";
 		const string XAMARIN_ANDROID_INSTALLER_KEY = "PrivateAndroidSdkPath";
 
+		string ndkstack = "ndk-stack.exe";
+
 		public override string Adb { get { return "adb.exe"; } }
 		public override string Android { get { return "android.bat"; } }
 		public override string Emulator { get { return "emulator.exe"; } }
 		public override string Monitor { get { return "monitor.bat"; } }
 		public override string ZipAlign { get { return "zipalign.exe"; } }
 		public override string JarSigner { get { return "jarsigner.exe"; } }
 		public override string KeyTool { get { return "keytool.exe"; } }
-		public override string NdkStack { get { return "ndk-stack.exe"; } }
+		public override string NdkStack { get { return ndkstack; } }
 		public override string NdkHostPlatform32Bit { get { return "windows"; } }
 		public override string NdkHostPlatform64Bit { get { return "windows-x86_64"; } }
 		public override string Javac { get { return "javac.exe"; } }
@@ -52,6 +54,17 @@ public override string PreferedJavaSdkPath {
 			}
 		}
 
+		public override void Initialize (string androidSdkPath = null, string androidNdkPath = null, string javaSdkPath = null)
+		{
+			base.Initialize (androidSdkPath, androidNdkPath, javaSdkPath);
+			// we need to look for extensions other than the default .exe|.bat
+			// google have a habbit of changing them.
+			foreach (var exe in Executables (NdkStack)) {
+				if (File.Exists (Path.Combine (AndroidNdkPath, exe)))
+					ndkstack = exe;
+			}
+		}
+
 		protected override IEnumerable<string> GetAllAvailableAndroidSdks ()
 		{
 			var roots = new[] { RegistryEx.CurrentUser, RegistryEx.LocalMachine };