[Battery Monitor] Don't blink when battery is critical

Especially when you're almost out of juice you don't want to be wasting it blinking an icon causing excess repaints.
Also considering the critical percentage is hardcoded to 5% here and doesn't follow PowerDevil's setting.
Given battery critical notification is persistent and always on top of everything these days,
you can't really miss the fact that you're almost ouf of power.

Differential Revision: https://phabricator.kde.org/D26434