* fix(large-video): do not show avatar if no url

By default the large video dominant speaker avatar
has an empty src, which will result in a broken
image displaying. There is also disconnect with
non-react code trying to set an undefined src.
To prevent such until local avatar generation
work is done in the future, just don't show the
avatar.

* fix(conference): set the room instance earlier

Set the room instance on APP.conference before triggering
a redux update of the conference being set,, because
middleware can then fire and call methods on APP.conference
that depend on the room being set.

* get local participant directly from store instead of from global