Context in dotnet#910

When switching to `xabuild.exe`, two mistakes were made:
- `xabuild.exe` shouldn’t set `AndroidSdkDirectory` or
`AndroidNdkDirectory`
- Xamarin.Android.Build.Tests should pass these values to xabuild

This restores some of the code in `Builder.cs` removed in f9d15dd

Context: https://github.com/jonathanpeppers/xabuild

What do we want? A *usable*, *parallel*, build tree.

To elaborate: we want to be able to have a "system" Xamarin.Android
install, and a "parallel" xamarin-android install, and be able to
*easily* switch between the two. (For various definitions of "easy";
here, we mean *command-line* use, not IDE use.)

On macOS, this (more or less) Just Works™, and is extremely handy
for testing bug fixes:

	$ xbuild /t:Install Project.csproj
	# Verify that some bug is triggered

	$ xbuild /t:Uninstall Project.csproj
	$ tools/scripts/xabuild /t:Install Project.csproj
	# Verify that some bug is *fixed*

There's One Problem™ with this: MSBuild does not make this easy.
(Related: commit aa1db83.) Apps may rely on files located within
`$(MSBuildExtensionsPath)` or `$(TargetFrameworkRootPath)`, files
such as PCL profile assemblies, or 3rd party frameworks.

Meanwhile, on macOS, `xabuild` is *predicated* upon overriding
`$(TargetFrameworkRootPath)` and `$(MSBuildExtensionsPath)` and
`$(XBUILD_FRAMEWORK_FOLDERS_PATH)`, and creating a bunch of symlinks
to "fake out" `msbuild.exe` so that system-installed files such as
PCL assemblies can be found *through* the parallel environment.

It kinda/sorta works on macOS. It completely falls apart when using
Windows. There is no easy "symlink half the world" solution there.
Overriding `$(MSBuildExtensionsPath)` means that
`Microsoft.Common.targets` can't be found. Overriding
`$(TargetFrameworkRootPath)` means PCL files can't be found.

It's a mess.

Fortunately, more recent versions of MSBuild allow for some of these
properties to contain *multiple* directories instead of a single
directory, which means *there is a way* to support our desired
usable parallel install world order. We "just" need to e.g.
force `$(MSBuildExtensionsPath)` to contain *both* the in-tree
directory *and* the system directory:

```xml
<MSBuildExtensionsPath>In-Tree Directory; System Directory</MSBuildExtensionsPath>
```

Unfortunately, *this isn't easy*. Not all of these properties can be
overridden on the `msbuild.exe` command line. Worse, MSBuild doesn't
allow `;` to be part of a property value, as `;` is a property name
[separator char](https://msdn.microsoft.com/en-us/library/ms164311.aspx)

	msbuild.exe /property:WarningLevel=2;OutDir=bin\Debug

The way to force MSBuild to accept multiple paths in a property value
is by providing a `.exe.config` file with the appropriate values.

~~ Enter `xabuild.exe` ~~

`xabuild.exe` is a nice wrapper around MSBuild for compiling
Xamarin.Android projects using a locally built version of
Xamarin.Android on your system. It seems to work on Windows, macOS, and
Linux and doesn’t require elevation or modifications to your system.

`xabuild.exe` works by doing the following:

 1. Reference `MSBuild.exe` or `MSBuild.dll` depending on the platform
 2. Creates symbolic links to `.NETPortable` and `.NETFramework`
    directories inside the Xamarin.Android build output directory
 3. Overrides MSBuild's `app.config` file to set various properties,
    such as `$(MSBuildExtensionsPath)`.
 4. Run MSBuild’s `Main()` method

~~ Usage ~~

On macOS, `tools/scripts/xabuild` has been updated to use
`xabuild.exe` when `$MSBUILD` is `msbuild:

	$ MSBUILD=msbuild tools/scripts/xabuild /t:SignAndroidPackage samples/HelloWorld/HelloWorld.csproj

When `$MSBUILD` is `xbuild` (the current default), the previous
behavior of overriding `$MSBuildExtensionsPath` and
`$XBUILD_FRAMEWORK_FOLDERS_PATH` is still used.

On Windows, MSBuild 15.3 from Visual Studio 2017 is required.
Simply execute `xabuild.exe`:

	> bin\Debug\bin\xabuild.exe Xamarin.Android-Tests.sln /p:XAIntegratedTests=False

Before `xabuild.exe` existed, `setup-windows.exe` would need to be
executed (as Administrator!) in order for `Xamarin.Android-Tests.sln`
to be built using `msbuild.exe`.