Added an auditallow rule to track vold remounting filesystems.

Vold shouldn't have this selinux permission, so this will be left in for
a few weeks to keep track of if removing it would be an issue to any
other processes. If not, then a follow-up CL will remove both the rule
and the auditallow

Test: This CL is a test in itself, auditallow rules shouldn't change
behavior of SELinux policy by themselves
Bug: 26901147
Change-Id: Ib076448863bd54278df59a3b514c9e877eb22ee5