Implement full-text search.

- Display thumbnails along with page results
- Highlight the term(s) that were searched
- Continue the search when results scrolled down
- Automatically re-search based on suggestion (in case of misspelling)
- Enable this functionality only if it's a non-production build (for now)

Bug: 68438
Change-Id: I9627539762e286f630ef1cea6ae193a042a0977d