Stop double-loading device admins

loadSettingsLocked() was being called twice for USER_OWNER at
systemReady(), doubling the number of admin entries at every boot.

Also guards against double-adding admins which appear twice in the
same XML file, favouring more recently-refreshed entries for
backward compatibility.

Previously an application calling in with 'refreshing = true' would
double-insert whatever admin was being refreshed into its user's list
of device admins. This is fixed too.

@bug 16416936

Change-Id: Idd147aa130e6bce7bcc40532f0a7fb07117b3151