Allow the system user to read apex_data_file.

For consistency with APKs, signature verification is performed
in the system_server. This includes checking that the signature of
an updated install matches the signature of the active package that
it updates. For this, it requires search access to /data/apex and
read access to the files under that directory.

Test: m
Change-Id: I8795b26b9a40ba7126c2a548fbec82ff322a1453