[Xamarin.Android.Build.Tasks] Set PROGUARD_HOME on Windows (#550)

Context: d71dcc9

The `$PROGUARD_HOME` environment variable is used by the
`<CreateMultiDexMainDexClassList/>` task/`mainDexClasses` script to
specify the location of the `proguard` directory+script to use.

`$PROGUARD_HOME` is exported on Linux and macOS but not Windows
because the `proguard.bat` distributed with the Android SDK contained
a quoting bug:

	java -jar "%PROGUARD_HOME%"\lib\proguard.jar %*

Starting with commit 59aff45, xamarin-android bundles its own
ProGuard binaries, which doesn't suffer from the above quoting bug.
Additionally, we want to use our bundled ProGuard, which necessitates
exporting the `%PROGUARD_HOME%` environment variable on Windows.

Export the `$PROGUARD_HOME` environment variable on all platforms.

Author: pjcollins

src/Xamarin.Android.Build.Tasks/Utilities/MonoAndroidHelper.cs

@@ -470,12 +470,13 @@ public static Dictionary<string, string> LoadAcwMapFile (string acwPath)
 
 		public static string [] GetProguardEnvironmentVaribles (string proguardHome)
 		{
+			string proguardHomeVariable = "PROGUARD_HOME=" + proguardHome;
+
 			return Environment.OSVersion.Platform == PlatformID.Unix ?
-				new string [] { "PROGUARD_HOME=" + proguardHome } :
-				//TODO ReAdd the PROGUARD_HOME env variable once we are shipping our own proguard
+				new string [] { proguardHomeVariable } :
 				// Windows seems to need special care, needs JAVA_TOOL_OPTIONS.
 				// On the other hand, xbuild has a bug and fails to parse '=' in the value, so we skip JAVA_TOOL_OPTIONS on Mono runtime.
-				new string [] { "JAVA_TOOL_OPTIONS=-Dfile.encoding=UTF8" };
+				new string [] { proguardHomeVariable, "JAVA_TOOL_OPTIONS=-Dfile.encoding=UTF8" };
 		}
 	}
 }