Check that dir name is a pid before attempting to read

Prevents selinux denials for folders in /proc that do not have the
default /proc label.

Bug: 68146208
Test: no selinux denials for vold attempting to read proc_asound dir.
Merged-In: I7cdd3bbe8e687e078372012773e9a34a5c76e0f8
Change-Id: I7cdd3bbe8e687e078372012773e9a34a5c76e0f8
1 file changed