monitor AC status rather than battery status

If you set a charging threshold on the battery, you may bounce between
charging and discharging while still on AC. The profile only needs to
change when AC is disconnected. If the battery is discharging, but AC is
connected, don't switch to a power-saving profile.

I think.

roles/laptop/files/power_monitor.sh

@@ -5,6 +5,7 @@
 BAT=$(echo /sys/class/power_supply/BAT*)
 BAT_STATUS="$BAT/status"
 BAT_CAP="$BAT/capacity"
+AC_STATUS=/sys/class/power_supply/ACAD/online
 LOW_BAT_PERCENT=50
 
 AC_PROFILE="desktop"
@@ -19,7 +20,7 @@ prev=0
 
 while true; do
     # read the current state
-    if [[ $(cat "$BAT_STATUS") == "Discharging" ]]; then
+    if [[ $(cat "$AC_STATUS") == "0" ]]; then
         if [[ $(cat "$BAT_CAP") -gt $LOW_BAT_PERCENT ]]; then
             profile=$BAT_PROFILE
         else
@@ -39,5 +40,5 @@ while true; do
     prev=$profile
 
     # wait for the next power change event
-    inotifywait -qq "$BAT_STATUS" "$BAT_CAP"
+    inotifywait -qq "$AC_STATUS" "$BAT_CAP"
 done