Add direct System.Memory reference to AndroidTestApplication.

src/Android/Avalonia.AndroidTestApplication/Avalonia.AndroidTestApplication.csproj

@@ -151,4 +151,5 @@
   <Import Project="$(MSBuildExtensionsPath)\Xamarin\Android\Xamarin.Android.CSharp.targets" />
   <Import Project="..\..\..\build\Serilog.props" />
   <Import Project="..\..\..\build\Rx.props" />
+  <Import Project="..\..\..\build\System.Memory.props" />
 </Project>