Post `onPostExecute` serially after posting `doInBackground` on ShadowAsyncTask if the background scheduler is paused

This fixes #686 in the standard Robolectric setup (where both the background and UI schedulers are paused). There is still a problem if
the UI thread is not paused but the background is. In this scenario the `onPostExecute` code will be run synchronously inside the task future's
`done` method and it will swallow any exceptions (as the Future has 'completed' technically).