allow oemfs:dir search

mediaserver and drmserver both have permission to read oemfs
related files. However, there are no search permissions on the
directory, so the files would be unreachable.

Grant search permissions on the oemfs directory, so that the files
within that directory can be read.

Bug: 17954291
Change-Id: I9e36dc7b940bd46774753c1fa07b0f47c36ff0db