VS for Mac Multi-target support (#953)

Should now be able to pull code, open sln, and build correctly.

Xamarin.Essentials/Xamarin.Essentials.csproj

@@ -76,8 +76,10 @@
     <AndroidResource Include="Resources\xml\*.xml" />
     <PackageReference Include="Xamarin.Android.Support.Compat" Version="28.0.0.3" />
     <PackageReference Include="Xamarin.Android.Support.CustomTabs" Version="28.0.0.3" />
-    <PackageReference Condition="$(TargetFrameworkVersion.TrimStart('vV')) &lt; 9.0" Include="Xamarin.Android.Support.Core.Utils" Version="28.0.0.3" />
+    <PackageReference Condition=" '$(OS)' == 'Windows_NT' And $(TargetFrameworkVersion.TrimStart('vV')) &lt; 9.0" Include="Xamarin.Android.Support.Core.Utils" Version="28.0.0.3" />
+    <PackageReference Condition=" '$(OS)' != 'Windows_NT' " Include="Xamarin.Android.Support.Core.Utils" Version="28.0.0.3" />
   </ItemGroup>
+
   <ItemGroup Condition=" $(TargetFramework.StartsWith('Xamarin.iOS')) ">
     <Compile Include="**\*.ios.cs" />
     <Compile Include="**\*.ios.*.cs" />

nuget.config

@@ -10,7 +10,6 @@
 
     <packageSources>
         <add key="NuGet official package source" value="https://api.nuget.org/v3/index.json" />
-        <add key="SDK Extras" value="https://www.myget.org/F/msbuildsdkextras/api/v3/index.json" />
     </packageSources>
 
 </configuration>