[macOS] fix DirectoryNotFoundException on clean systems (#110)

@PureWeen was hitting a problem building a .NET 6 app on macOS:

	Task "ResolveSdks"
	  ...
	  at System.IO.Enumeration.FileSystemEnumerator`1.CreateDirectoryHandle(String path, Boolean ignoreNotFound)
	  at System.IO.Enumeration.FileSystemEnumerator`1.Init()
	  at System.IO.Enumeration.FileSystemEnumerator`1..ctor(String directory, Boolean isNormalized, EnumerationOptions options)
	  at System.IO.Enumeration.FileSystemEnumerable`1..ctor(String directory, FindTransform transform, EnumerationOptions options, Boolean isNormalized)
	  at System.IO.Enumeration.FileSystemEnumerableFactory.UserDirectories(String directory, String expression, EnumerationOptions options)
	  at System.IO.Directory.InternalEnumeratePaths(String path, String searchPattern, SearchTarget searchTarget, EnumerationOptions options)
	  at Xamarin.Android.Tools.AndroidSdkBase.FindBestNDK(String androidSdkPath)
	  at Xamarin.Android.Tools.AndroidSdkBase.GetAllAvailableAndroidNdks()+MoveNext()
	  at Xamarin.Android.Tools.AndroidSdkBase.GetValidNdkPath(String ctorParam)
	  at Xamarin.Android.Tools.AndroidSdkBase.Initialize(String androidSdkPath, String androidNdkPath, String javaSdkPath)
	  at Xamarin.Android.Tools.AndroidSdkInfo..ctor(Action`2 logger, String androidSdkPath, String androidNdkPath, String javaSdkPath)
	  at Xamarin.Android.Tasks.MonoAndroidHelper.RefreshAndroidSdk(String sdkPath, String ndkPath, String javaPath, TaskLoggingHelper logHelper)
	  at Xamarin.Android.Tasks.ResolveSdks.RunTask()
	  at Microsoft.Android.Build.Tasks.AndroidTask.Execute()

This was a relatively new Mac which didn't have an Android NDK
installed.

A way to workaround the problem was to create a directory that was
being checked for:

	mkdir -p ~/Library/Android/sdk

It appears this problem was introduced in b2d9fdf, as:

 1. `~/Library/Android/sdk` is returned from
    [`GetAllAvailableAndroidNdks()`][0]


 2. `GetAllAvailableAndroidNdks()` calls `GetAllAvailableAndroidSdks()`
    looking for any `ndk-bundle` folders it can find.

 3. `FindBestNDK()` is passed a directory that doesn't exist, and the
    `Directory.EnumerateDirectories()` call [throw][1].

The fix here is to update `FindBestNDK()` to check `Directory.Exists()`
before trying to traverse the directory `androidSdkPath`, as the values
from `GetSdkFromEnvironmentVariables()` may not exist either.

I was able to add a test for this scenario by adding an invalid
directory to `$ANDROID_HOME`.

[0]: https://github.com/xamarin/xamarin-android-tools/blob/b2d9fdf8782875ffb63da48cf8e0b385b04f7c25/src/Xamarin.Android.Tools.AndroidSdk/Sdks/AndroidSdkUnix.cs#L113-L115
[1]: https://github.com/xamarin/xamarin-android-tools/blob/b2d9fdf8782875ffb63da48cf8e0b385b04f7c25/src/Xamarin.Android.Tools.AndroidSdk/Sdks/AndroidSdkBase.cs#L172

Author: jonathanpeppers

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

@@ -152,7 +152,7 @@ protected virtual IEnumerable<string> GetAllAvailableAndroidNdks ()
 			// Look in PATH
 			foreach (var ndkStack in ProcessUtils.FindExecutablesInPath (NdkStack)) {
 				var ndkDir  = Path.GetDirectoryName (ndkStack);
-				if (ndkDir == null)
+				if (string.IsNullOrEmpty (ndkDir))
 					continue;
 				yield return ndkDir;
 			}
@@ -161,12 +161,19 @@ protected virtual IEnumerable<string> GetAllAvailableAndroidNdks ()
 			foreach (var sdk in GetAllAvailableAndroidSdks ()) {
 				if (sdk == AndroidSdkPath)
 					continue;
-				yield return FindBestNDK (sdk);
+				var ndkDir = FindBestNDK (sdk);
+				if (string.IsNullOrEmpty (ndkDir))
+					continue;
+				yield return ndkDir;
 			}
 		}
 
 		string FindBestNDK (string androidSdkPath)
 		{
+			if (!Directory.Exists (androidSdkPath)) {
+				return String.Empty;
+			}
+
 			var ndkInstances = new SortedDictionary<Version, string> (Comparer<Version>.Create ((Version l, Version r) => r.CompareTo (l)));
 
 			foreach (string ndkPath in Directory.EnumerateDirectories (androidSdkPath, "ndk*", SearchOption.TopDirectoryOnly)) {

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

@@ -146,6 +146,28 @@ public void Ndk_PathInSdk()
 			}
 		}
 
+		[Test]
+		public void Ndk_AndroidSdkDoesNotExist ()
+		{
+			CreateSdks (out string root, out string jdk, out string ndk, out string sdk);
+
+			Action<TraceLevel, string> logger = (level, message) => {
+				Console.WriteLine ($"[{level}] {message}");
+				if (level == TraceLevel.Error)
+					Assert.Fail (message);
+			};
+
+			var oldAndroidHome = Environment.GetEnvironmentVariable ("ANDROID_HOME");
+			try {
+				Environment.SetEnvironmentVariable ("ANDROID_HOME", "/i/dont/exist");
+				// Check that this doesn't throw
+				new AndroidSdkInfo (logger, androidSdkPath: sdk, androidNdkPath: null, javaSdkPath: jdk);
+			} finally {
+				Environment.SetEnvironmentVariable ("ANDROID_HOME", oldAndroidHome);
+				Directory.Delete (root, recursive: true);
+			}
+		}
+
 		[Test]
 		public void Constructor_SetValuesFromPath ()
 		{