backend: Reset idletime when unplugging the power cable

Avoid having laptops suspend or lock as soon as the power cable is
unplugged as the timeout for those actions when on battery are smaller
than the timeouts when on AC.

- laptop is plugged in, and hasn't been used for X minutes
- laptop is unplugged
- the gnome-settings-daemon power plugin sets up its timeouts for
  inactivity for the "on battery" case
- those X minutes of inactivity are still counting, and are above
  the level of one of the timeouts (say, suspend or lock screen),
  mutter fires the timeouts
- gsd-power activates the action associated with the timeout

Fixes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1953
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2029>

src/backends/meta-backend.c

@@ -181,6 +181,7 @@ struct _MetaBackendPrivate
   guint upower_watch_id;
   GDBusProxy *upower_proxy;
   gboolean lid_is_closed;
+  gboolean on_battery;
 
   guint sleep_signal_id;
   GCancellable *cancellable;
@@ -670,6 +671,23 @@ upower_properties_changed (GDBusProxy *proxy,
         }
     }
 
+  v = g_variant_lookup_value (changed_properties,
+                              "OnBattery",
+                              G_VARIANT_TYPE_BOOLEAN);
+  if (v)
+    {
+      gboolean on_battery;
+
+      on_battery = g_variant_get_boolean (v);
+      g_variant_unref (v);
+
+      if (on_battery != priv->on_battery)
+        {
+          priv->on_battery = on_battery;
+          reset_idle_time = TRUE;
+        }
+    }
+
   if (reset_idle_time)
     meta_idle_manager_reset_idle_time (priv->idle_manager);
 }
@@ -713,6 +731,13 @@ upower_ready_cb (GObject      *source_object,
                          priv->lid_is_closed);
         }
     }
+
+  v = g_dbus_proxy_get_cached_property (proxy, "OnBattery");
+  if (v)
+    {
+      priv->on_battery = g_variant_get_boolean (v);
+      g_variant_unref (v);
+    }
 }
 
 static void