[UnindexedFileIndexer] Only mark file for content indexing when needed

Summary:
In case the index level is set to "non content indexing", the file was
still flagged when it had changed. As the content indexer checks for
the level itself (the config may have changed in the meantime), the
effect is limited, but the code is still wrong.

In case the level is set to "content indexing", the flag would be set
twice. As it is a flag the effect is limited, but it blows up the
transaction unnecessarily.

Reviewers: Baloo, ngraham, davidedmundson

Reviewed By: Baloo, ngraham, davidedmundson

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D28326