Add resource dlls to MicroBuild signing. (#145)

BaRS is conducting an effort to migrate signing in Xamarin pipelines off
the [external Groovy pipeline][0]. The goal is to use MicroBuild to
handle all signing within the build rather than hand off files for
another pipeline to sign. The [Android.SDK.Manager][1] pipeline has been
chosen as the first pipeline to migrate. Project files in
`Android.SDK.Manager` reference the [xamarin-android-tools repo][2] as a
submodule and [sign the resource dlls][3] that the
`Xamarin.Android.Tools.AndroidSdk` project creates. This change will
move the signing of these resource dlls to the build of
`Xamarin.Android.Tools.AndroidSdk`. The added `<GetFilesToSign/>` target
enables the use of wildcards in MicroBuild.

This change is not intended to impact the behavior of the `AndroidTools`
build beyond the added signing.

[0]: https://dev.azure.com/devdiv/DevDiv/_build?definitionId=13878&_a=summary
[1]: https://dev.azure.com/devdiv/DevDiv/_build?definitionId=13502&_a=summary
[2]: https://github.com/xamarin/android-sdk-installer/blob/684b7e1283706bef2e5c832a64ced0158abe5259/Xamarin.Installer.AndroidSDK.Manager/Xamarin.Installer.AndroidSDK.Manager.csproj#L154
[3]: https://dev.azure.com/devdiv/DevDiv/_build/results?buildId=5479174&view=logs&j=7082d35a-e46b-5a74-0b0f-e46ed113ca0e&t=9ebd6855-a6bc-5b9a-0813-711468e28365&l=91

Author: mattnorflus

src/Xamarin.Android.Tools.AndroidSdk/Xamarin.Android.Tools.AndroidSdk.csproj

@@ -38,11 +38,16 @@
     <PackageReference Include="XliffTasks" Version="1.0.0-beta.20420.1" PrivateAssets="all" />
   </ItemGroup>
 
-  <ItemGroup>
-    <FilesToSign Include="$(OutDir)\$(AssemblyName).dll">
-      <Authenticode>Microsoft400</Authenticode>
-    </FilesToSign>
-  </ItemGroup>
+  <Target Name="GetFilesToSign" BeforeTargets="SignFiles">
+    <ItemGroup>
+      <FilesToSign Include="$(OutDir)\$(AssemblyName).dll">
+        <Authenticode>Microsoft400</Authenticode>
+      </FilesToSign>
+      <FilesToSign Include="$(OutDir)\**\$(AssemblyName).resources.dll">
+        <Authenticode>Microsoft400</Authenticode>
+      </FilesToSign>
+    </ItemGroup>
+  </Target>
 
   <ItemGroup>
     <Compile Update="Properties\Resources.Designer.cs">