Added the permission for MANAGE_USERS in the priv_app allowlist

This is necessary to allow NetworkStack to identify whether
a connection was setup trough an enterprise-managed configuration,
by querying DevicePolicyManager. Some behavior specific to
enterprise networks require NetworkStack to differentiate this case.

Bug: 189865016
Change-Id: Iaeb91ebf9cccce8bf630fab5d1883473247b4def