FileBrowser: fix sort race condition at resume

On some devices, when first fetching the dataset for the browser,
browseImpl will be called during the reSort that is called by
onResume. This means that the reSort starts with a empty dataset,
browseImpl populates it, and reSort finishes by reassigning an
empty set to dataset.value.

In on Resume if the dataset is empty then it will be populated and
sorted by browseImpl, if it is not, then it has already been
populated, but possibly with the wrong sort as resetSort hasn't
been called. Hence in this case reSort should be called just in case