[native, web, lib] [refactor] [ENG-536] move search index to hook

Summary: moves the search index code that's used in both native and web into lib/hooks

Test Plan: test sidebar search functionality in both native/ and web they should both work as expected.

Reviewers: atul, palys-swm, def-au1t

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4056