refactor(display): Output/layer/battery thread safety.

* Submit widget updates to display queue.
* Use mutex to control access to shared state for widgets.

app/src/display/main.c

@@ -22,6 +22,7 @@ LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
 #define ZMK_DISPLAY_NAME CONFIG_LVGL_DISPLAY_DEV_NAME
 
 static const struct device *display;
+static bool initialized = false;
 
 static lv_obj_t *screen;
 
@@ -82,6 +83,8 @@ static void stop_display_updates() {
     k_timer_stop(&display_timer);
 }
 
+int zmk_display_is_initialized() { return initialized; }
+
 int zmk_display_init() {
     LOG_DBG("");
 
@@ -108,6 +111,8 @@ int zmk_display_init() {
 
     start_display_updates();
 
+    initialized = true;
+
     LOG_DBG("");
     return 0;
 }