GetAllAvailableAndroidSdks on macOS should also include the default Xamarin installation folder

[sgmunn]

If a user does not have a preferred Android SDK path (ie, ~/.config/xbuild/monodroid-config.xml is missing), then we fail to find the installed android sdk because we do not check the default location that it is installed to.

~/Library/Developer/Xamarin/android-sdk-macosx

https://github.com/xamarin/xamarin-android-tools/blob/master/src/Xamarin.Android.Tools.AndroidSdk/Sdks/AndroidSdkUnix.cs#L106

Can we include that folder as well?