fix(sxmo): re-enable automatic screen-off on idle to stop backlight battery drain

The previous commit disabled sxmo_idle.sh and the idle_locker in the
unlock/lock hooks, leaving the display backlight pinned at maximum
brightness forever (Xorg runs with -s 0 -dpms and /sys/power/state is
empty, so the panel never blanks). On a phone with no suspend this is by
far the dominant battery drain.

Restore sxmo_idle.sh and start idle_locker from the unlock and lock hooks
so the screen blanks after SXMO_UNLOCK_IDLE_TIME (default 30s) / once
locked after SXMO_LOCK_IDLE_TIME (default 8s). Locking itself stays
explicit-only, matching the documented design; only the display blanks.

Verified live on the device: after the idle timeout the state transitions
to screenoff and /sys/class/backlight/backlight/brightness drops to 0.

overlay/rootfs/usr/bin/sxmo_idle.sh

@@ -1,4 +1,78 @@
 #!/bin/sh
 # SPDX-License-Identifier: AGPL-3.0-only
-# Automatic idle locking and screen blanking are disabled on Bananarchy.
-exit 0
+# Copyright 2022 Sxmo Contributors
+
+_swayidletoidles() {
+	while [ $# -gt 0 ]; do
+		case "$1" in
+			timeout)
+				printf "%s|%s" "$2" "$3"
+				shift 3
+				case "$1" in
+					resume)
+						printf "|%s" "$2"
+						shift 2
+				;;
+				esac
+				printf "\n"
+				;;
+			*)
+				shift
+				;;
+		esac
+	done
+}
+
+xorgidle() {
+	if ! command -v xprintidle >/dev/null 2>&1; then
+		while : ; do
+			sleep 3600
+		done
+	fi
+
+	idles="$(_swayidletoidles "$@")"
+	resumes=""
+
+	tick=0
+	new_idle="$(xprintidle 2>/dev/null || printf '0')"
+	[ -n "$new_idle" ] || new_idle=0
+	last_idle="$new_idle"
+
+	finish() {
+		sh -c "$resumes"
+		resumes=""
+		exit
+	}
+	trap 'finish' TERM INT EXIT
+
+	while : ; do
+		last_idle="$new_idle"
+		new_idle="$(xprintidle 2>/dev/null || printf '0')"
+		[ -n "$new_idle" ] || new_idle=0
+		if [ "$last_idle" -gt "$new_idle" ] 2>/dev/null; then
+			sh -c "$resumes"
+			tick=0
+			resumes=""
+		fi
+
+		if printf "%b\n" "$idles" | grep -q "^$tick|"; then
+			printf "%b\n" "$idles" | \
+				grep "^$tick|" | \
+				cut -d'|' -f2 | \
+				xargs -I{} -0 sh -c "{}"
+			resumes="$(printf "%b\n" "$idles" | grep "^$tick|" | cut -d'|' -f3);$resumes"
+		fi
+
+		sleep 1
+		tick=$((tick + 1))
+	done
+}
+
+case "$SXMO_WM" in
+	dwm|i3)
+		xorgidle "$@"
+		;;
+	sway|river)
+		exec swayidle "$@"
+		;;
+esac