feature(display): Blank display on idle/sleep.

* Set display blanking, and stop refresh timer for displays
  when the activity state goes to idle/sleep, and resume
  when transitioning to active again.

app/src/display/main.c

@@ -14,6 +14,8 @@ LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
 #include <drivers/display.h>
 #include <lvgl.h>
 
+#include <zmk/event-manager.h>
+#include <zmk/events/activity-state-changed.h>
 #include <zmk/display/status_screen.h>
 
 #define ZMK_DISPLAY_NAME CONFIG_LVGL_DISPLAY_DEV_NAME
@@ -35,6 +37,18 @@ void display_timer_cb() { k_work_submit(&display_tick_work); }
 
 K_TIMER_DEFINE(display_timer, display_timer_cb, NULL);
 
+static void start_display_updates() {
+    display_blanking_off(display);
+
+    k_timer_start(&display_timer, K_MSEC(10), K_MSEC(10));
+}
+
+static void stop_display_updates() {
+    display_blanking_on(display);
+
+    k_timer_stop(&display_timer);
+}
+
 int zmk_display_init() {
     LOG_DBG("");
 
@@ -54,10 +68,29 @@ int zmk_display_init() {
     lv_scr_load(screen);
 
     lv_task_handler();
-    display_blanking_off(display);
 
-    k_timer_start(&display_timer, K_MSEC(10), K_MSEC(10));
+    start_display_updates();
 
     LOG_DBG("");
     return 0;
-}
+}
+
+int display_event_handler(const struct zmk_event_header *eh) {
+    struct activity_state_changed *ev = cast_activity_state_changed(eh);
+    switch (ev->state) {
+    case ZMK_ACTIVITY_ACTIVE:
+        start_display_updates();
+        break;
+    case ZMK_ACTIVITY_IDLE:
+    case ZMK_ACTIVITY_SLEEP:
+        stop_display_updates();
+        break;
+    default:
+        LOG_WRN("Unhandled activity state: %d", ev->state);
+        return -EINVAL;
+    }
+    return 0;
+}
+
+ZMK_LISTENER(display, display_event_handler);
+ZMK_SUBSCRIPTION(display, activity_state_changed);