[Xamarin.Android.Build.Tasks] Add build dependency on BootstrapTasks.

Add a build-time @(ProjectReference) on the
Xamarin.Android.Tools.BootstrapTasks project so that
Xamarin.Android.Tools.BootstrapTasks is rebuilt if necessary.

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Build.Tasks.csproj

@@ -629,6 +629,11 @@
       <Name>Mono.Android</Name>
       <ReferenceOutputAssembly>False</ReferenceOutputAssembly>
     </ProjectReference>
+    <ProjectReference Include="..\Xamarin.Android.Tools.BootstrapTasks\Xamarin.Android.Tools.BootstrapTasks.csproj">
+      <Project>{E8492EFB-D14A-4F32-AA28-88848322ECEA}</Project>
+      <Name>Xamarin.Android.Tools.BootstrapTasks</Name>
+      <ReferenceOutputAssembly>False</ReferenceOutputAssembly>
+    </ProjectReference>
   </ItemGroup>
   <ItemGroup>
     <Folder Include="pdb2mdb\" />