Refine search logic a bit.

- No longer offer the choice of Title vs. Full-text search.
- Default to Title search, but if it returns fewer than 5 results, then
  switch to Full-text search seamlessly.
- Switch back to Title search when a new search term is given.
- Once we're convinced that this method is better than before, we can
  refactor the whole thing so that there's only one sub-fragment (instead
  of Title vs. Full).

Change-Id: Ibf9ebc49299393a0afc17b87b4b8bc02f0ba74a6