Check for ANDROID_HOME Sdk location on Windows. (#230)

For some reason we never check for the `%ANDROID_HOME%` environment
variable on Windows.  This might cause issues with people using VSCode
since we probably will be using environment variables as a backup when
using that IDE.

Add a check to use `%ANDROID_HOME%` after checking the registry
entries.  This way Visual Studio and Android SDK Installer paths will
still be preferred.

Author: dellis1972

src/Xamarin.Android.Tools.AndroidSdk/Sdks/AndroidSdkWindows.cs

@@ -93,6 +93,9 @@ protected override IEnumerable<string> GetAllAvailableAndroidSdks ()
 				if (CheckRegistryKeyForExecutable (root, ANDROID_INSTALLER_PATH, ANDROID_INSTALLER_KEY, wow, "platform-tools", Adb))
 					yield return RegistryEx.GetValueString (root, ANDROID_INSTALLER_PATH, ANDROID_INSTALLER_KEY, wow) ?? "";
 
+			foreach (string dir in GetSdkFromEnvironmentVariables ())
+				yield return dir;
+
 			// Check some hardcoded paths for good measure
 			var paths = new string [] {
 				Path.Combine (Environment.GetFolderPath (Environment.SpecialFolder.LocalApplicationData), "Xamarin", "MonoAndroid", "android-sdk-windows"),