Deprecate overriding SearchEngine::search*

Deprecate overriding SearchEngine::search*

The plan is to convert these methods into final, considering
it a removal under the deprecation policy. By making entry
points into the search engine final we provide a guaranteed
point where generic handling can be applied to all search engines.

The first use case for this generic handling is pushing pagination
via overfetch into the SearchEngine class instead of re-implementing
an overfetch in individual parts of the code that perform searches.

Change-Id: I3426d6a2f32d8b368b044b154e1cb70dac007c62