Allow vendor_init to getattr vold_metadata_file.

This relaxes the neverallow rule blocking vendor_init from doing
anything to vold_metadata_file.  The rules above it still prevent it
from doing anything other than relabelto and getattr.

Bug: 79681561
Test: Boot device and see no denials.
Change-Id: I1beb25bb9f8d69323c9fee53a140c2a084b12124