問題点
This issue happens when you are trying to use fast deployment on a device which does not support it. Fast deployment requires features which are not available on devices running API 20 or lower. The Fast Deployment system makes use of the run-as feature of the Android OS. This feature was either not available or had limited capabilities in API 20 and earlier.
解決策
Disable Fast Deployment by setting EmbedAssembliesIntoApk = True in your .csproj.
または、IDE で Fast Deployment をオフにします。 デバイスでは引き続きデバッグできます。必要なすべてのファイルが.apk内にパッケージ化されます。