LogoSystem: fix loading images in widget
LogoSystem: fix loading images in widget

Don't load the image at _init, because at this point
we really don't know what mode we will end-up using.
Therefore, load the image only when the uri property
is set.

https://phabricator.endlessm.com/T14121

endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib