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))
 yieldreturnRegistryEx.GetValueString(root,ANDROID_INSTALLER_PATH,ANDROID_INSTALLER_KEY,wow)??"";
 
+foreach(stringdirinGetSdkFromEnvironmentVariables())
+yieldreturndir;
+
 // Check some hardcoded paths for good measure
 varpaths=newstring[]{
 Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),"Xamarin","MonoAndroid","android-sdk-windows"),