[Microsoft.Android.Build] Bump to MSBuild 16.10.0 (#125)

Related: dotnet/android#6014

Drops the OS specific MSBuild NuGet and system references and instead
uses MSBuild 16.10.0 NuGet packages.  This introduces an issue in our
usage of `Microsoft.Build.Construction.ProjectRootElement.Create` in
xamarin/xamarin-android on macOS, however we can work around it there.

Author: pjcollins

src/Microsoft.Android.Build.BaseTasks/MSBuildReferences.projitems

@@ -4,49 +4,20 @@
 <Project>
   <!--Import this file in projects needing to reference Microsoft.Build.*.dll -->
   <PropertyGroup>
-    <MSBuildReferenceVersion>15.1.0.0</MSBuildReferenceVersion>
-    <MSBuildPackageReferenceVersion>16.5</MSBuildPackageReferenceVersion>
+    <MSBuildPackageReferenceVersion>16.10.0</MSBuildPackageReferenceVersion>
     <LibZipSharpVersion>2.0.0-alpha6</LibZipSharpVersion>
     <MonoUnixVersion>7.0.0-alpha8.21302.6</MonoUnixVersion>
   </PropertyGroup>
 
-  <ItemGroup Condition="$([MSBuild]::IsOSPlatform('windows'))" >
-    <!-- Only use these on Windows as it causes problems with running unit tests in the IDE's on MacOS -->
+  <ItemGroup>
     <PackageReference Include="Microsoft.Build"                Version="$(MSBuildPackageReferenceVersion)" />
     <PackageReference Include="Microsoft.Build.Framework"      Version="$(MSBuildPackageReferenceVersion)" />
     <PackageReference Include="Microsoft.Build.Tasks.Core"     Version="$(MSBuildPackageReferenceVersion)" />
     <PackageReference Include="Microsoft.Build.Utilities.Core" Version="$(MSBuildPackageReferenceVersion)" />
-  </ItemGroup>
-
-  <ItemGroup Condition="!$([MSBuild]::IsOSPlatform('windows'))">
-    <Reference Include="Microsoft.Build, Version=$(MSBuildReferenceVersion), Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" >
-      <SpecificVersion>True</SpecificVersion>
-      <HintPath>$(MSBuildToolsPath)\Microsoft.Build.dll</HintPath>
-    </Reference>
-    <Reference Include="Microsoft.Build.Framework, Version=$(MSBuildReferenceVersion), Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" >
-      <SpecificVersion>True</SpecificVersion>
-      <HintPath>$(MSBuildToolsPath)\Microsoft.Build.Framework.dll</HintPath>
-    </Reference>
-    <Reference Include="Microsoft.Build.Tasks.Core, Version=$(MSBuildReferenceVersion), Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" >
-      <SpecificVersion>True</SpecificVersion>
-      <HintPath>$(MSBuildToolsPath)\Microsoft.Build.Tasks.Core.dll</HintPath>
-    </Reference>
-    <Reference Include="Microsoft.Build.Utilities.Core, Version=$(MSBuildReferenceVersion), Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" >
-      <SpecificVersion>True</SpecificVersion>
-      <HintPath>$(MSBuildToolsPath)\Microsoft.Build.Utilities.Core.dll</HintPath>
-    </Reference>
-  </ItemGroup>
-
-  <ItemGroup>
     <PackageReference Include="K4os.Compression.LZ4" Version="1.1.11" />
     <PackageReference Include="Xamarin.Build.AsyncTask" Version="0.3.4" GeneratePathProperty="true" />
     <PackageReference Include="Xamarin.LibZipSharp" Version="$(LibZipSharpVersion)" GeneratePathProperty="true" />
     <PackageReference Include="Mono.Unix" Version="$(MonoUnixVersion)" GeneratePathProperty="true" />
-    <PackageReference Include="System.Buffers" Version="4.5.1" />
-    <PackageReference Include="System.Collections.Immutable" Version="1.7.1" />
-    <PackageReference Include="System.Numerics.Vectors" Version="4.5.0" />
-    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="4.7.1" />
-    <PackageReference Include="System.ValueTuple" Version="4.5.0" />
   </ItemGroup>
 
 </Project>