[Bookmarks Runner] Initialize Chrome favicon query only once per profile

Summary:
The db scheme will not change during the FetchSqlite livetime. Query
the scheme just once per krunner invocation instead of once per result.

Test Plan: make

Reviewers: Frameworks, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D15489