[UnindexedFileIndexer] Skip filetime checks for new files

Summary:
In case a file is not in the index, either because it is new, or as it
should not be indexed, there is no need to retrieve the timestamps from
the file (it will be different).

Reviewers: Baloo, ngraham, astippich, poboiko

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D21577