Set new icon for battery applet

Summary: It's not visible in the desktop system tray, but it is visible in the Plasma Mobile drawer where it is the only colorful icon. Let's use a monochrome one that actually looks like a battery instead

Reviewers: #plasma, bshah, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24649