FileIndexingJob: Pause it when on battery

The FileIndexingJob normally just takes one iteration, except in the few
exceptional cases when there is an offending file. In those cases, we
try to call the extractor again with half those files, and keep
splitting until we find the offending file.

Normally, the file indexer is never run when on battery, so this patch
just makes sure the error detection code is not run when on battery and
the file indexer was already running.