Silently drop unknown titles in completion search

Silently drop unknown titles in completion search

This mimics how full text works by silenty dropping results returned from
search that no longer exist. This could be because the search index is slightly
out of sync with reality, or the search engine could simply be broken.

Only silent from the users perspective. We maintain a count in statsd of
the number of titles dropped. This can be monitored over time to
recognize any increases.

Bug: T115756
Change-Id: I2f29d73e258cd448a14d35a2b4902a4fb6f61c68