Fix getById for files in appdata and the root mount

In case the path we are currently in is inside the appdata_* folder,
the original getById method does not work, because it can only look inside
the user's mount points. But the user has no mount point for the root storage.

So in that case we directly check the mount of the root if it contains
the id. If it does we check if the path is inside the path we are working
in.

Signed-off-by: Joas Schilling <coding@schilljs.com>