search: Always fetch the list of search providers

We fetch and store the list of providers from the search system when we
construct SearchResults, but we never update this list when providers are
changed at runtime, causing various bugs making the search not seem as
snappy as it should be. Make sure to always fetch the list of providers
from the search system.
viewSelector: Move all of the provider-loading logic to SearchSystem

The existing provider system is split between a confusing mess of
RemoteSearch, SearchSystem, SearchDisplay, and ViewSelector, partly
because of the vestigal in-shell search system. Move most of the
logic to search.js so it's easier to read.