Bug 1018240 - Part 1: reinitialize nsSearchService when the browser locale changes. r=adw

This patch makes nsSearchService watch for changes in the
general.useragent.locale pref, asynchronously reiniting itself when the pref
changes.

reinit-complete and reinit-failed events are broadcast on the usual search
service topic.
Bug 1018240 - Part 2: invalidate BrowserSearch engine list when locale has changed. r=bnicholson