[PowerStateMonitor] Be conservative when determining power state

Summary:
When the state defaults to AC-powered, the indexer may start some
energy consuming tasks, only to stop these later when the pending
DBus call finishes. Especially the content indexer can take a while
to stop, until the current batch is finished.

In case the DBus call fails AC-powered is assumed, to match the previous
default.

Test Plan:
unplug AC power
start baloo_file
-> the content indexer no longer processes its first batch

Reviewers: Baloo, ngraham, astippich, poboiko

Reviewed By: poboiko

Subscribers: kde-frameworks-devel, Baloo

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D21672