[FileIndexScheduler] Cleanup firstRun handling

Summary:
firstRun is an internal state of baloo, there is no need to write it to
the config. We can determine it from the existence of the DB file. In
case the DB is only populated partially, the remainder will be picked
up by the UnnindexedFileIndexer.

Delay the content indexing during startup. Currently, we only delay
the UnindexedFileIndexer and StaleFileIndexer, but files already flagged
for indexing would be picked up immediately.

Apply the same delay also to the initial run.

Test Plan:
A) First run:
kill baloo_file manually, delete the index
start baloo_file -> 5 second delay

B) Login
kill baloo_file manually
touch some files
start baloo_file -> content indexing delayed

Reviewers: Baloo, ngraham

Reviewed By: Baloo, ngraham

Subscribers: broulik, kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D27923