Allow domain to getattr on apex_mnt_dir

The dynamic linker calls realpath(3) on paths found in the linker config
script. Since realpath() calls lstat() on the parent paths, not allowing
getattr on /apex and its subdirectories will cause selinux denial spam
whenever something is executed from APEXes.

Silence the spam by allowing getattr on apex_mnt_dir.

Bug: 117403679
Bug: 115787633
Test: m apex.test; m; device is bootable

Change-Id: Ic659582760a3ae146e73770266bc64332b36a97c