I installed Android Studio on the same PC as Xamarin. I wanted to use the same adb as Visual Studio in Android Studio.
(android-sdk path is an option on the initial install screen, but I had to use mklink to get around the fact that VS installs Android sdk at Program Files (x86) - Android Studio doesn't like non alphanumeric characters.
When I returned to Xamarin I could no longer compile, this seemed to be the main error: C:\Program Files (x86)\MSBuild\Xamarin\Android\Xamarin.Android.Common.targets(1183,2): error MSB6006: "aapt.exe" exited with code -1073741819.