phone(dconf): Suspend on battery only

Suspend *significantly* increases the battery life of the phone at the
cost of not receiving internet-based notifications.

hosts/phone/configuration.nix

@@ -60,6 +60,11 @@ in
           image-gallery = true;
           sidebar = false;
         };
+
+        "org/gnome/settings-daemon/plugins/power" = {
+          sleep-inactive-ac-type = "nothing";
+          sleep-inactive-battery-type = "suspend";
+        };
       };
     };
   };