[Xamarin.Android.Build.Tasks] Use correct Mono.Android.dll

We're trying to get [`make jenkins`][0] working on Jenkins, and
[it's failing][1], as one might expect when a particular repo and
associated build system has never been run on Jenkins before:

	Android.App/ApplicationTest.cs(9,7): error CS0246: The type or namespace name `NUnit' could not be found. Are you missing an assembly reference?

This error occurs while building
`src/Mono.Android/Test/Mono.Android-Tests.csproj`, and happens because
the `Xamarin.Android.NUnitLite.dll` assembly isn't referenced...
because it isn't *found*:

	Microsoft.Common.targets: warning : Reference 'Xamarin.Android.NUnitLite' not resolved

This raises a long-standing issue with building `xamarin-android`:
The Xamarin.Android SDK is intended to be usable from a system-wide
installation environment, e.g. installed into
`$(MSBuildExtensionsPath)`, as many MSBuild tasks such as
`<GetReferenceAssemblyPaths/>` are used to resolve assemblies, and
those -- by default -- look in system-wide installation locations.

"Unfortunately," A `xamarin-android` *build* **should not** be built
into the system-wide install location; it should be built in a
separate location, for all manner of "hygiene" and "sanity" reasons.

Additionally, overriding `$(TargetFrameworkRootPath)` isn't a reliable
solution.

All of which adds up to either build failures such as the above --
when there is no system-wide Xamarin.Android install -- or the
use/resolution of the system-wide Xamarin.Android assemblies when
building e.g. `src/Mono.Android/Test`, which is wholly undesirable.

A possible "fix"/"workaround": *force* `xbuild` to behave as we want
it to by setting the `$MSBuildExtensionsPath` and
`$XBUILD_FRAMEWORK_FOLDERS_PATH` environment variables
*from within the build process*. Since `xbuild` doesn't cache the
value of these environment variables, calling
`Environment.SetEnvironmentVariable()` within the build process will
cause subsequent `<GetReferenceAssemblyPaths/>`/etc. invocations to
use the updated values, allowing us to control which directories are
used for assembly framework resolution purposes.

Update the `_GetReferenceAssemblyPaths` target within
`Xamarin.Android.Common.targets` and
`Xamarin.Android.Bindings.targets` so that they call the new
`<SetMSBuildExtensionsPath/>` task, which updates the
`$MSBuildExtensionsPath` and `$XBUILD_FRAMEWORK_FOLDERS_PATH`
environment variables. This allows building `src/Mono.Android/Test`
with "normal" `xbuild` to use the intended assemblies without
requiring the use of `tools/scripts/xabuild` to build
`src/Mono.Android/Test`.

[0]: a16673d
[1]: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android/20/console

Author: jonpryor

src/Xamarin.Android.Build.Tasks/Tasks/SetMSBuildExtensionsPath.cs

@@ -0,0 +1,49 @@
+using System;
+using System.IO;
+using System.Linq;
+
+using Microsoft.Build.Utilities;
+using Microsoft.Build.Framework;
+
+namespace Xamarin.Android.Tasks
+{
+	public sealed class SetMSBuildExtensionsPath : Task
+	{
+		const   string      MSBuildExtensionsPath           = "MSBuildExtensionsPath";
+		const   string      XBUILD_FRAMEWORK_FOLDERS_PATH   = "XBUILD_FRAMEWORK_FOLDERS_PATH";
+		public override bool Execute ()
+		{
+			var frameworksPath  = Path.GetDirectoryName (typeof (SetMSBuildExtensionsPath).Assembly.Location);
+			if (Path.DirectorySeparatorChar == '\\') {
+				// TODO: Default Windows search location
+			} else {
+				// e == $prefix/lib/xbuild/Xamarin/Android
+				// Want: $prefix/lib/xbuild-frameworks
+				if (!frameworksPath.EndsWith ("xbuild/Xamarin/Android", StringComparison.OrdinalIgnoreCase)) {
+					throw new NotSupportedException ("Cannot determine path to xbuild-frameworks!");
+				}
+				frameworksPath  = Path.GetDirectoryName (Path.GetDirectoryName (Path.GetDirectoryName (frameworksPath)));
+				frameworksPath  = Path.Combine (frameworksPath, "xbuild-frameworks");
+			}
+
+			UpdateEnvironmentVariable (MSBuildExtensionsPath,           frameworksPath);
+			UpdateEnvironmentVariable (XBUILD_FRAMEWORK_FOLDERS_PATH,   frameworksPath);
+
+			return !Log.HasLoggedErrors;
+		}
+
+		void UpdateEnvironmentVariable (string environmentVariable, string newPath)
+		{
+			var p   = (Environment.GetEnvironmentVariable (environmentVariable) ?? "")
+				.Split (new [] { Path.PathSeparator }, StringSplitOptions.RemoveEmptyEntries);
+			if (p.Any (x => string.Equals (x, newPath, StringComparison.OrdinalIgnoreCase))) {
+				return;
+			}
+
+			var newValue    = string.Join (Path.PathSeparator.ToString (), new [] { newPath }.Concat (p));
+			Log.LogMessage (MessageImportance.Low, $"  Setting environment variable `{environmentVariable}`='{newValue}'.");
+			Environment.SetEnvironmentVariable (environmentVariable, newValue);
+		}
+	}
+}
+

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Bindings.targets

@@ -35,6 +35,7 @@ Copyright (C) 2012 Xamarin Inc. All rights reserved.
   <UsingTask TaskName="Xamarin.Android.Tasks.CreateNativeLibraryArchive" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
   <UsingTask TaskName="Xamarin.Android.Tasks.ImportJavaDoc" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
   <UsingTask TaskName="Xamarin.Android.Tasks.MDoc" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
+  <UsingTask TaskName="Xamarin.Android.Tasks.SetMSBuildExtensionsPath" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 
   <!--
   *******************************************
@@ -142,6 +143,7 @@ Copyright (C) 2012 Xamarin Inc. All rights reserved.
   </PropertyGroup>
 
 <Target Name="_GetReferenceAssemblyPaths">
+	<SetMSBuildExtensionsPath />
 	<GetReferenceAssemblyPaths
 			TargetFrameworkMoniker="$(TargetFrameworkIdentifier),Version=v1.0"
 			RootPath="$(TargetFrameworkRootPath)">

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Build.Tasks.csproj

@@ -391,6 +391,7 @@
     <Compile Include="Tasks\CheckTargetFrameworks.cs" />
     <Compile Include="Tasks\Dx.cs" />
     <Compile Include="Tasks\CreateMsymManifest.cs" />
+    <Compile Include="Tasks\SetMSBuildExtensionsPath.cs" />
     <Compile Include="Utilities\AndroidResource.cs">
       <Link>Utilities\AndroidResource.cs</Link>
     </Compile>

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -91,6 +91,7 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
 <UsingTask TaskName="Xamarin.Android.Tasks.Proguard" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.DetermineJavaLibrariesToCompile" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.CreateMultiDexMainDexClassList" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
+<UsingTask TaskName="Xamarin.Android.Tasks.SetMSBuildExtensionsPath" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 
 <!--
 *******************************************
@@ -531,6 +532,7 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
 </Target>
 
 <Target Name="_GetReferenceAssemblyPaths">
+	<SetMSBuildExtensionsPath />
 	<GetReferenceAssemblyPaths
 			TargetFrameworkMoniker="$(TargetFrameworkIdentifier),Version=v1.0"
 			RootPath="$(TargetFrameworkRootPath)">