Commit bd18313d12 changed to a new naming scheme for battery icons,
and used to old icon names as fallback-icon-name for compatibility
with older/other icon themes.
However that fallback code isn't working correctly, as GThemedIcon's
default fallbacks will transform a name of `battery-level-90-symbolic`
to a list of names:
- `battery-level-90-symbolic`
- `battery-level-symbolic`
- `battery-symbolic`
The last one frequently exists, so instead of the intended fallback,
we end up with a generic battery icon.
Address this by specifying the icon as GIcon instead of an icon-name,
where we have more control over how the icon is resolved.
https://gitlab.gnome.org/GNOME/gnome-shell/issues/1442
bd5162105e