Include web search providers in Searchables.

- Along with ACTION_SEARCH we now enumate ACTION_WEB_SEARCH as well so web search providers are covered in the searchables list. This fixes a broken unit test.
- Moved get/setPreferredWebSearchActivity and get-all-web-search-providers implementation to this module when the searchables list gets updated, so that it happens on boot and on package add/remove events and remains up to date. The duplicate code in WebSearchProvider will be removed in a separate change.
- Also made Searchables broadcast an intent when the searchables list got rebuilt, so components such as GlobalSearch/SuggestionSources no longer need to do this on their own.