[create-vsix] Fix build ordering for xa-prep-tasks on Windows (#836)

Context: https://devdiv.visualstudio.com/DevDiv/Default/_build/index?buildId=978440&_a=summary

On Windows the following scenario is happening:

  - `_CreateDependencies` (and `GetXAVersionInfo`) load xa-prep-tasks.dll
  - This locks the assembly (on Windows only)
  - `<ProjectReference/>` causes xa-prep-tasks to build, but the output
    assembly is locked
  - There is also an issue here if xa-prep-tasks is not built yet, as
    MSBuild tasks are used from it.

The fix is to add a `DependsOnTargets` for `ResolveAssemblies`, this
builds xa-prep-tasks before using it.

build-tools/create-vsix/create-vsix.targets

@@ -99,7 +99,7 @@
       Returns="$(IsExperimental)">
   </Target>
   <Target Name="_CreateDependencies"
-      DependsOnTargets="GetXAVersionInfo"
+      DependsOnTargets="ResolveReferences;GetXAVersionInfo"
       BeforeTargets="Build"
       Inputs="Xamarin.Android.Sdk.pkgdef.in"
       Outputs="Xamarin.Android.Sdk.pkgdef">