favicon.js: use setAndFetchFaviconForPage for xulrunner 18 and above

nsIFaviconService::setAndLoadFaviconForPage was marked deprecated in
favor of mozIAsyncFavicons::setAndFetchFaviconForPage in xulrunner 21.
Since we already have a branch for xulrunner 18+ when the privacy
argument was added, and since favicon_service implements
mozIAsyncFavicons at least as far back as 18, I'm reusing that branch.