Fix sort ordering for ndk-bundle, add macOS support

Context: https://build.azdo.io/xamarin/public/21545

An environment change on Azure DevOps seems to cause the test failure
on Windows:

    Ndk_PathInSdk
    AndroidNdkPath not found inside sdk!
    Expected string length 71 but was 53. Strings differ at index 3.
    Expected: "C:\Users\VssAdministrator\AppData\Local\Temp\tmpAE78.tmp\sdk\..."
    But was: "C:\Program Files (x86)\Android\android-sdk\ndk-bundle"

It appears that `xamarin-android-tools` is preferring an `ndk-bundle
found on the system instead of the value passed in for
`AndroidSdkPath`.

To make matters worse, use of `Distinct()` returns an unordered
sequence, and so the sort ordering is unknown:

https://docs.microsoft.com/dotnet/api/system.linq.enumerable.distinct

To fix this, let's not use `ToList()` or add `AndroidSdkPath` to the
list at all. We can make an explicit check for it before the `foreach`
loop. Inside the loop we should skip `AndroidSdkPath`.

I also added the same code for the macOS/linux side in
`AndroidSdkUnix`.

I updated the `Ndk_PathInSdk` test so it will run on all platforms.

Author: jonathanpeppers

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

@@ -125,6 +125,24 @@ protected override IEnumerable<string> GetAllAvailableAndroidNdks ()
 				if (ValidateAndroidNdkLocation (ndkDir))
 					yield return ndkDir;
 			}
+
+			// Check for the "ndk-bundle" directory inside the SDK directories
+			string ndk;
+
+			if (!string.IsNullOrEmpty (AndroidSdkPath) &&
+				Directory.Exists (ndk = Path.Combine (AndroidSdkPath, "ndk-bundle"))) {
+				if (ValidateAndroidNdkLocation (ndk))
+					yield return ndk;
+			}
+
+			foreach (var sdk in GetAllAvailableAndroidSdks ()) {
+				if (sdk == AndroidSdkPath)
+					continue;
+				if (Directory.Exists (ndk = Path.Combine (sdk, "ndk-bundle"))) {
+					if (ValidateAndroidNdkLocation (ndk))
+						yield return ndk;
+				}
+			}
 		}
 
 		protected override string GetShortFormPath (string path)

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

@@ -232,14 +232,20 @@ protected override IEnumerable<string> GetAllAvailableAndroidNdks ()
 			// Check for the "ndk-bundle" directory inside the SDK directories
 			string ndk;
 
-			var sdks = GetAllAvailableAndroidSdks().ToList();
-			if (!string.IsNullOrEmpty(AndroidSdkPath))
-				sdks.Add (AndroidSdkPath!);
-	
-			foreach(var sdk in sdks.Distinct())
-				if (Directory.Exists(ndk = Path.Combine(sdk, "ndk-bundle")))
-					if (ValidateAndroidNdkLocation(ndk))
+			if (!string.IsNullOrEmpty (AndroidSdkPath) &&
+					Directory.Exists (ndk = Path.Combine (AndroidSdkPath, "ndk-bundle"))) {
+				if (ValidateAndroidNdkLocation (ndk))
+					yield return ndk;
+			}
+
+			foreach (var sdk in GetAllAvailableAndroidSdks ()) {
+				if (sdk == AndroidSdkPath)
+					continue;
+				if (Directory.Exists (ndk = Path.Combine (sdk, "ndk-bundle"))) {
+					if (ValidateAndroidNdkLocation (ndk))
 						yield return ndk;
+				}
+			}
 
 			// Check for the key the user gave us in the VS/addin options
 			foreach (var root in roots)

tests/Xamarin.Android.Tools.AndroidSdk-Tests/AndroidSdkInfoTests.cs

@@ -67,29 +67,31 @@ public void Constructor_Paths ()
 		[Test]
 		public void Ndk_PathInSdk()
 		{
-			if (!OS.IsWindows)
-				Assert.Ignore("This only works in Windows");
-
 			CreateSdks(out string root, out string jdk, out string ndk, out string sdk);
 
 			var logs = new StringWriter();
 			Action<TraceLevel, string> logger = (level, message) => {
 				logs.WriteLine($"[{level}] {message}");
 			};
 
+			var oldPath = Environment.GetEnvironmentVariable ("PATH", EnvironmentVariableTarget.Process);
 			try
 			{
+				Environment.SetEnvironmentVariable ("PATH", "", EnvironmentVariableTarget.Process);
+
+				var extension = OS.IsWindows ? ".cmd" : "";
 				var ndkPath = Path.Combine(sdk, "ndk-bundle");
 				Directory.CreateDirectory(ndkPath);
 				Directory.CreateDirectory(Path.Combine(ndkPath, "toolchains"));
-				File.WriteAllText(Path.Combine(ndkPath, "ndk-stack.cmd"), "");
+				File.WriteAllText(Path.Combine(ndkPath, $"ndk-stack{extension}"), "");
 
 				var info = new AndroidSdkInfo(logger, androidSdkPath: sdk, androidNdkPath: null, javaSdkPath: jdk);
 
 				Assert.AreEqual(ndkPath, info.AndroidNdkPath, "AndroidNdkPath not found inside sdk!");
 			}
 			finally
 			{
+				Environment.SetEnvironmentVariable ("PATH", oldPath, EnvironmentVariableTarget.Process);
 				Directory.Delete(root, recursive: true);
 			}
 		}