DRTVWR-363: Fix LLNotificationsListener::listChannels() channel walk.

LLNotifications::ChannelMap went away when LLNotificationChannel became an
LLInstanceTracker subclass. Iterate the universe of channels using
LLNotificationChannel::beginInstances(), endInstances() instead.
More troubling is that LLNotificationChannel::getParentChannelName() went away
too. When LLNotificationChannel acquired a Params block and corresponding
constructor, it acquired the ability to listen on multiple upstream sources.
That meant that a single mParent string became inapplicable, and its access
method was removed. (Curiously, mParent was not itself removed, but it was
left unused.) Change mParent to mParents, a vector<string>, built by
connectToChannel(). Introduce getParents(), an accessor returning an
iterator_range over that vector.
Change LLNotificationsListener::listChannels() to collect a "parents" key in
the map returned for each channel, and -- for backwards compatibility --
capture the first entry in the "parents" array as "parent".