Allow apexd to realpath(3) on apex_key_files

apexd uses realpath(3) to ensure that the public key file that will use
is under /system/etc/security/apex directory. In order to support it,
allow apexd to getattr on apex_key_files.

The canonicalization is required because the key name from APEX might be
wrong. For example, if the key name from an APEX is '../../some/path'
then apexd will use '/system/etc/security/apex/../../some/path' as the
public key file, which is incorrect.

Bug: 115721587
Test: m apex.test; m
/apex/com.android.example.apex@1 exists

Change-Id: I6dc5efa0de369f8497e4f6526e0164e2de589c67