Allow init to rm /dev/diag

Commit 69e1ad839d8a89f55eb226a639c760ac09e7135a
(AOSP cherrypick 3ac5654c0a144eda4925c70e5c2f275e95c31e7c) ensures
that /dev/diag is always removed on boot. Allow for it in
SELinux policy.

Addresses the following denial:

  audit(1422745424.741:5): avc:  denied  { unlink } for  pid=1 comm="init" name="diag" dev="tmpfs" ino=8302 scontext=u:r:init:s0 tcontext=u:object_r:diag_device:s0 tclass=chr_file

Change-Id: If20ae7eb64356c06e94873dec89fc1ca576fe74a