[Sample] [Android] Fixed PagesGallery.Droid build (#3999)

* fixed PagesGallery.Droid build

* Moved Xamarin.Forms.Platform.Android (Forwarders) reference to previous place

* updated project guid to lower case

PagesGallery/PagesGallery.Droid/PagesGallery.Droid.csproj

@@ -179,7 +179,7 @@
   <ItemGroup>
     <ProjectReference Include="..\..\Stubs\Xamarin.Forms.Platform.Android\Xamarin.Forms.Platform.Android %28Forwarders%29.csproj">
       <Project>{6e53feb1-1100-46ae-8013-17bba35cc197}</Project>
-      <Name>Xamarin.Forms.Platform.Android %28Forwarders%29</Name>
+      <Name>Xamarin.Forms.Platform.Android (Forwarders)</Name>
     </ProjectReference>
     <ProjectReference Include="..\..\Xamarin.Forms.Core\Xamarin.Forms.Core.csproj">
       <Project>{57b8b73d-c3b5-4c42-869e-7b2f17d354ac}</Project>