[tests] fix failures related to AndroidSdkDirectory property (#977)

The introduction of `xabuild` in commit f9d15dd omitted a design
requirement: `xabuild` needs to be runnable on a machine which does
*does not* have a xamarin-android build environment. The idea is that
e.g. Windows developers could download a Jenkins-built
`oss-xamarin.android*.zip` file, extract it, and run
`bin\Debug\bin\xabuild.exe` to use the extract artifacts *without*
requiring system-wide installation via `setup-windows.exe`.

This not-very-well-stated requirement brought along two mistakes:

 1. `xabuild.exe` should *not* set the `$(AndroidSdkDirectory)` and
    `$(AndroidNdkDirectory)` MSBuild properties. It *can't*; it could
    be executed on ~any machine, in particular one which hasn't built
    xamarin-android, so setting these properties to "build-tree"
    values doesn't make sense and will fail.

 2. Even if (1) weren't the case, commit f9d15dd still allowed the
    `Xamarin.Android.Build.Tests` unit tests to use `xbuild` as the
    MSBuild engine, via the `tools/scripts/xabuild` script.
    This script *does not* always set `$(AndroidSdkDirectory)` or
    `$(AndroidNdkDirectory)`.
    
As a result of (2), many unit tests *fail*:

	Error executing task ResolveSdks: System.InvalidOperationException: Could not determine Android SDK location. Please provide `androidSdkPath`.
	  at Xamarin.Android.Tools.AndroidSdkInfo..ctor (System.Action`2[T1,T2] logger, System.String androidSdkPath, System.String androidNdkPath, System.String javaSdkPath) [0x0008e] in …/xamarin-android/external/xamarin-android-tools/src/Xamarin.Android.Tools.AndroidSdk/AndroidSdkInfo.cs:28 
	  at Xamarin.Android.Tasks.MonoAndroidHelper.RefreshAndroidSdk (System.String sdkPath, System.String ndkPath, System.String javaPath) [0x00021] in …/xamarin-android/src/Xamarin.Android.Build.Tasks/Utilities/MonoAndroidHelper.cs:111 
	  at Xamarin.Android.Tasks.ResolveSdks.RunTask () [0x001a0] in …/xamarin-android/src/Xamarin.Android.Build.Tasks/Tasks/ResolveSdksTask.cs:157 
	  at Xamarin.Android.Tasks.ResolveSdks.Execute () [0x00002] in …/xamarin-android/src/Xamarin.Android.Build.Tasks/Tasks/ResolveSdksTask.cs:130 
	  at Microsoft.Build.BuildEngine.TaskEngine.Execute () [0x00000] in …/mcs/class/Microsoft.Build.Engine/Microsoft.Build.BuildEngine/TaskEngine.cs:134 
	  at Microsoft.Build.BuildEngine.BuildTask.Execute () [0x0008d] in …/mcs/class/Microsoft.Build.Engine/Microsoft.Build.BuildEngine/BuildTask.cs:101 

Update `xabuild` to *never* explicitly set `$(AndroidSdkDirectory)`
and `$(AndroidNdkDirectory)`, and update `Xamarin.Android.Build.Tests`
to *always* explicitly provide these MSBuild properties. This allows
unit test execution to be more reliable, and will prevent 
`xabuild.exe` from looking at "random" non-existent directories.

Author: jonathanpeppers

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Common/Builder.cs

@@ -185,6 +185,19 @@ protected bool BuildInternal (string projectOrSolution, string target, string []
 					buildLogFullPath, Verbosity.ToString ().ToLower ());
 
 			var start = DateTime.UtcNow;
+			var homeDirectory = Environment.GetFolderPath (Environment.SpecialFolder.Personal);
+ 			var androidSdkToolPath = Path.Combine (homeDirectory, "android-toolchain");
+ 			var sdkPath = Environment.GetEnvironmentVariable ("ANDROID_SDK_PATH");
+ 			if (String.IsNullOrEmpty (sdkPath))
+ 				sdkPath = GetPathFromRegistry ("AndroidSdkDirectory");
+ 			if (String.IsNullOrEmpty (sdkPath))
+ 				sdkPath = Path.GetFullPath (Path.Combine (androidSdkToolPath, "sdk"));
+ 			var ndkPath = Environment.GetEnvironmentVariable ("ANDROID_NDK_PATH");
+ 			if (String.IsNullOrEmpty (ndkPath))
+ 				ndkPath = GetPathFromRegistry ("AndroidNdkDirectory");
+ 			if (String.IsNullOrEmpty (ndkPath))
+ 				ndkPath = Path.GetFullPath (Path.Combine (androidSdkToolPath, "ndk"));
+
 			var args  = new StringBuilder ();
 			var psi   = new ProcessStartInfo (XABuildExe);
 			args.AppendFormat ("{0} /t:{1} {2}",
@@ -193,6 +206,12 @@ protected bool BuildInternal (string projectOrSolution, string target, string []
 				args.Append (" /p:BuildingOutOfProcess=true");
 			else
 				args.Append (" /p:UseHostCompilerIfAvailable=false /p:BuildingInsideVisualStudio=true");
+			if (Directory.Exists (sdkPath)) {
+				args.AppendFormat (" /p:AndroidSdkDirectory=\"{0}\" ", sdkPath);
+			}
+			if (Directory.Exists (ndkPath)) {
+				args.AppendFormat (" /p:AndroidNdkDirectory=\"{0}\" ", ndkPath);
+			}
 			if (parameters != null) {
 				foreach (var param in parameters) {
 					args.AppendFormat (" /p:{0}", param);

tools/xabuild/XABuild.cs

@@ -64,8 +64,6 @@ static XmlDocument CreateConfig (XABuildPaths paths)
 			SetProperty (toolsets, "MSBuildExtensionsPath", paths.MSBuildExtensionsPath);
 			SetProperty (toolsets, "MSBuildExtensionsPath32", paths.MSBuildExtensionsPath);
 			SetProperty (toolsets, "RoslynTargetsPath", Path.Combine (paths.MSBuildBin, "Roslyn"));
-			SetProperty (toolsets, "AndroidSdkDirectory", paths.AndroidSdkDirectory);
-			SetProperty (toolsets, "AndroidNdkDirectory", paths.AndroidNdkDirectory);
 			SetProperty (toolsets, "MonoAndroidToolsDirectory", paths.MonoAndroidToolsDirectory);
 			SetProperty (toolsets, "TargetFrameworkRootPath", paths.FrameworksDirectory + Path.DirectorySeparatorChar); //NOTE: Must include trailing \
 

tools/xabuild/XABuildPaths.cs

@@ -83,10 +83,6 @@ class XABuildPaths
 
 		public string MonoAndroidToolsDirectory { get; private set; }
 
-		public string AndroidSdkDirectory { get; private set; }
-
-		public string AndroidNdkDirectory { get; private set; }
-
 		public XABuildPaths ()
 		{
 			IsWindows                 = Environment.OSVersion.Platform == PlatformID.Win32NT;
@@ -96,7 +92,6 @@ public XABuildPaths ()
 			XamarinAndroidBuildOutput = Path.GetFullPath (Path.Combine (XABuildDirectory, ".."));
 
 			string programFiles       = Environment.GetFolderPath (Environment.SpecialFolder.ProgramFilesX86);
-			string userProfile        = Environment.GetFolderPath (Environment.SpecialFolder.UserProfile);
 			string prefix             = Path.Combine (XamarinAndroidBuildOutput, "lib", "xamarin.android");
 
 			if (IsWindows) {
@@ -130,8 +125,6 @@ public XABuildPaths ()
 			FrameworksDirectory       = Path.Combine (prefix, "xbuild-frameworks");
 			MSBuildExtensionsPath     = Path.Combine (prefix, "xbuild");
 			MonoAndroidToolsDirectory = Path.Combine (prefix, "xbuild", "Xamarin", "Android");
-			AndroidSdkDirectory       = Path.Combine (userProfile, "android-toolchain", "sdk");
-			AndroidNdkDirectory       = Path.Combine (userProfile, "android-toolchain", "ndk");
 			MSBuildExeTempPath        = Path.GetTempFileName ();
 			XABuildConfig             = MSBuildExeTempPath + ".config";
 		}