fs_mgr: Strengthen ReadFstabFromFile() around gsi_public_metadata_file

ReadFstabFromFile() calls access() to check the existence of DSU
metadata files to determine if device is in DSU running state. This is
error prone because a failed access() can mean non-exsitent file as well
as the caller lacking the permission to path resolute the pathname.

Strengthen ReadFstabFromFile() to check the errno after a failed
access() or open(), if the errno is not ENOENT, then return with error,
as this may be indicating the caller doesn't have sufficient access
rights to call ReadFstabFromFile().

After this change, processes would need these policies to call
ReadFstabFromFile():
  allow scontext { metadata_file gsi_metadata_file_type }:dir search;
And these policies to call ReadFstabFromFile() within a DSU system:
  allow scontext gsi_public_metadata_file:file r_file_perms;

Bug: 181110285
Test: Presubmit
Change-Id: I1a6a796cb9b7b49af3aa5e7a5e8d99cde25e5857