Use %ProgramW6432% path for 32-bit apps (#75)

Context: https://docs.microsoft.com/en-us/windows/win32/winprog64/wow64-implementation-details#environment-variables

Use the `%ProgramW6432%` environment variable (if available) so that
both **Prefer 32-bit**=false and **Prefer 32-bit**=true apps can get
the path.  Otherwise `Environment.SpecialFolder.ProgramFiles` returns
`"C:\Program Files (x86)"` for 32-bit apps on 64-bit OS, which is not
where the Android SDK is installed, and thus path lookup fails.

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

@@ -93,7 +93,9 @@ protected override IEnumerable<string> GetAllAvailableAndroidSdks ()
 				Path.Combine (Environment.GetFolderPath (Environment.SpecialFolder.LocalApplicationData), "Xamarin", "MonoAndroid", "android-sdk-windows"),
 				Path.Combine (Environment.GetFolderPath (Environment.SpecialFolder.ProgramFilesX86), "Android", "android-sdk"),
 				Path.Combine (Environment.GetFolderPath (Environment.SpecialFolder.ProgramFilesX86), "Android", "android-sdk-windows"),
-				Path.Combine (Environment.GetFolderPath (Environment.SpecialFolder.ProgramFiles), "Android", "android-sdk"),
+				!string.IsNullOrEmpty (Environment.GetEnvironmentVariable ("ProgramW6432"))
+					? Path.Combine (Environment.GetEnvironmentVariable ("ProgramW6432"), "Android", "android-sdk")
+					: Path.Combine (Environment.GetFolderPath (Environment.SpecialFolder.ProgramFiles), "Android", "android-sdk"),
 				Path.Combine (Environment.GetFolderPath (Environment.SpecialFolder.LocalApplicationData), "Android", "android-sdk"),
 				Path.Combine (Environment.GetFolderPath (Environment.SpecialFolder.CommonApplicationData), "Android", "android-sdk"),
 				@"C:\android-sdk-windows"