Added a default channel for Leanplum push notifications as fallback to cover the cases where we receive
push notifications from server with an invalid channel and the case where there are no channels configured
on server.
Provided a valid default icon for push notifications in Leanplum required for fallback strategy.
Differential Revision: https://phabricator.services.mozilla.com/D4704
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extra : moz-landing-system : lando
