Fix for SMACK-841
Since Smack 4.2.4, the getPresencesInternal method in the Roster class
can return a LruCache object, which is a LinkedHashMap with access
order. This means that any access using get or getOrDefault will be a
modification of the Map. If you loop over the keySet of the Map and
there are more than one, the second call to get will throw a
ConcurrentModificationException!
Since the keys are only used here to obtain the corresponding
values, the simplest solution is to just loop over the values instead.
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