[incfs] fix the storage loading for existing mounts

IncrementalService used to keep the storage parameters in the
directory metadata, but our current incfs doesn't support
metadata on directories - so the storage id is encoded in its
name. But the loading code used to think it's still in the
metadata and couldn't load anything for that reason

Bug: 151241369
Test: install an app and reboot the phone. It's still there
Change-Id: Ic93f8f368b48fc5c5cc9bb726eff80478183596c