Usage of Collections.unmodifiableCollection is unsafe

Usage of Collections.unmodifiableCollection is unsafe

This is a follow-up to https://git.opendaylight.org/gerrit/#/c/51583/
wrt a review comment questioning why I returned a new ArrayList instead
of returning the Set directly. One reason was to avoid mutation of the
internal set by the caller but also to capture the state of the set at
that point in time and avoid concurrent mods which may not be safe.
Where a concurrent set is used, it would be thread-safe to return the
set directly but the set may by modified as the caller is iterating it
which may not be desired. For the other class whose set is a keySet
from a non-threadsafe Map, the caller could get a ConcurrentMod
exception while iterating. Based on the review, I changed the call
sites to return a Collections.unmodifiableCollection but this is also
incorrect and is susceptible to the same issues as that impl reads-thru
to the underlying collection. Therefore I changed the call sites back
to returning a new ArrayList.

Change-Id: I504f38c5bfc4c707180ac301eb10acd0ac24f872
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>