releasetools: Fix fstab path detection when input_file is a path to zip

If _FindAndLoadRecoveryFstab() is called with a path to target files zip
file, then it'll always attempt to read the fallback recovery.fstab
path. By making sure to pass zipfile.ZipFile(), it'll be able to read
the zip file ToC and use the correct path instead.

Test: Generate signed ota package for gts4lv
Change-Id: I4f3b975c677b7928999d0fe4fb137c868d7a206e