power: Use more fine-grained battery levels

Adwaita-icon-theme added new battery icons which represent battery levels
in 10% steps[0]. Use these if they are available, otherwise fall back to
the existing icon names for compatibility with older icon themes.

[0] GNOME/adwaita-icon-theme#6

GNOME/gnome-shell!561
4 jobs for !561 with fine-grained-battery-levels in 3 minutes and 49 seconds
detached
Status Job ID Name Coverage
  Review
passed #353041
check_commit_log

00:00:24

 
  Build
passed #353042
build

00:02:40

 
  Test
passed #353043
test

00:00:45

passed #353044
test-pot

00:00:44