Make use of FileHelpers.NormalizeFilePathSeparators

First part of the FileHelpers things for #2555

This is an easy one that makes a nice cleanup, so submitting it by itself.

The remaining FileHelpers are mostly just used by StorageDevice, so maybe we leave them for now as #2389 is going to tackle them.
If you do plan on using the code in there, ```DirectoryGetFiles``` and ```DirectoryGetDirectories``` both ignore the given path on WINDOWS_STOREAPP.
It looks like this bug was copy/pasted out of StorageDevice.
(I'll send a PR to fix them when I get a chance.)