[tests] fixed search paths for MSBuild on Windows (#869)

`Builder.cs` used in the unit tests does not currently use MSBuild from
Visual Studio 2017.

MSBuild is now located by checking the following:

- Check `%XA_MSBUILD_EXE%` environment variable
- Check VS 2017 path
- Check older VS path
- Check .NET path

Also updated `FrameworkLibDirectory` to check VS 2017 path.

Author: jonathanpeppers

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

@@ -14,7 +14,6 @@ public class Builder : IDisposable
 		const string fixed_linux_xbuild_path = "/usr/bin";
 		const string xbuildapp = "xbuild";
 		const string msbuildapp = "msbuild";
-		string msbuildExe;
 
 		public bool IsUnix { get; set; }
 		public bool RunningMSBuild { get; set; }
@@ -36,11 +35,57 @@ string GetUnixBuildExe ()
 			return File.Exists (path) ? path : msbuildapp;
 		}
 
+		string GetVisualStudio2017Directory ()
+		{
+			var editions = new [] {
+				"Enterprise",
+				"Professional",
+				"Community",
+				"BuildTools"
+			};
+
+			var x86 = Environment.GetFolderPath (Environment.SpecialFolder.ProgramFilesX86);
+			foreach (var edition in editions) {
+				var dir = Path.Combine (x86, "Microsoft Visual Studio", "2017", edition);
+				if (Directory.Exists (dir))
+					return dir;
+			}
+
+			return null;
+		}
+
+		string GetWindowsBuildExe ()
+		{
+			RunningMSBuild = true;
+
+			//First try environment variable
+			string msbuildExe = Environment.GetEnvironmentVariable ("XA_MSBUILD_EXE");
+			if (!string.IsNullOrEmpty (msbuildExe) && File.Exists (msbuildExe))
+				return msbuildExe;
+
+			//Next try VS 2017, MSBuild 15.0
+			var visualStudioDirectory = GetVisualStudio2017Directory ();
+			if (!string.IsNullOrEmpty(visualStudioDirectory)) {
+				msbuildExe = Path.Combine (visualStudioDirectory, "MSBuild", "15.0", "Bin", "MSBuild.exe");
+
+				if (File.Exists (msbuildExe))
+					return msbuildExe;
+			}
+
+			//Try older than VS 2017, MSBuild 14.0
+			msbuildExe = Path.Combine (Environment.GetFolderPath (Environment.SpecialFolder.ProgramFilesX86), "MSBuild", "14.0", "Bin", "MSBuild.exe");
+			if (File.Exists (msbuildExe))
+				return msbuildExe;
+			
+			//MSBuild 4.0 last resort
+			return Path.Combine (Environment.GetFolderPath (Environment.SpecialFolder.Windows), "Microsoft.NET", "Framework", "v4.0.30319", "MSBuild.exe");
+		}
+
 		public string MSBuildExe {
 			get {
 				return IsUnix
 					? GetUnixBuildExe ()
-					: msbuildExe;
+					: GetWindowsBuildExe ();
 			}
 		}
 
@@ -59,6 +104,10 @@ public string FrameworkLibDirectory {
 					return Path.Combine (outdir, "lib", "xamarin.android");
 				}
 				else {
+					var visualStudioDirectory = GetVisualStudio2017Directory ();
+					if (!string.IsNullOrEmpty (visualStudioDirectory))
+						return Path.Combine (visualStudioDirectory, "MSBuild", "Xamarin", "Android");
+
 					var x86 = Environment.GetFolderPath (Environment.SpecialFolder.ProgramFilesX86);
 					return Path.Combine (x86, "MSBuild", "Xamarin", "Android");
 				}
@@ -113,13 +162,6 @@ public Builder ()
 		{
 			IsUnix = Environment.OSVersion.Platform != PlatformID.Win32NT;
 			BuildLogFile = "build.log";
-			// Allow the override of the location of MSBuild and try a couple of backup paths for 
-			// MSBuild 14.0 and 4.0 
-			msbuildExe = Environment.GetEnvironmentVariable ("XA_MSBUILD_EXE");
-			if (String.IsNullOrEmpty (msbuildExe) || !File.Exists (msbuildExe))
-				msbuildExe = Path.Combine (Environment.GetFolderPath (Environment.SpecialFolder.ProgramFilesX86), "MSBuild", "14.0", "Bin", "MSBuild.exe");
-			if (!File.Exists (msbuildExe))
-				msbuildExe = string.Format ("{0}\\Microsoft.NET\\Framework\\v4.0.30319\\msbuild.exe", Environment.GetEnvironmentVariable ("WINDIR"));
 		}
 
 		public void Dispose ()