[FileIndexScheduler] Avoid emitting state changes multiple times

Summary:
FileContentIndexer::quit() will cause emission of the done() signal,
triggering scheduleIndexing() later. scheduleIndexing() sets the
current indexer state on its own and also emits the stateChanged signal,
there is no need to do it from the various slots.

Test Plan: unplug AC, state change signal is still emitted

Reviewers: Baloo, ngraham, astippich

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel, Baloo

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D21539