Problem:
solution:
A workaround is to copy your files to the old directory. Just copy the aapt.exe file from %android_sdk%uild-tools17.0.0 to%android_sdk%platform-tools.
And also copy the %android_sdk%uild-tools17.0.0lib folder to %android_sdk%platform-toolslib. There is 1 file in that directory, dx.jar which is also needed by Xamarin.
Everything should work normally. Not a really good way to fix the problem, but for now it is a good workaround. When Xamarin fixes this issue, it is a good idea to remove the dummy files
Reference
http://forums.xamarin.com/discussion/4180/aapt-exe-location-incorrect