refactor(power): Extract activity/idle detection.

* Refactor power to extract more general purpose
  activity detection/events.
* Use activity state to implement PM callback.

app/src/activity.c

@@ -0,0 +1,80 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <device.h>
+#include <init.h>
+#include <kernel.h>
+
+#include <logging/log.h>
+
+LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
+
+#include <zmk/event-manager.h>
+#include <zmk/events/activity-state-changed.h>
+#include <zmk/events/position-state-changed.h>
+#include <zmk/events/sensor-event.h>
+
+#include <zmk/activity.h>
+
+static enum zmk_activity_state activity_state;
+
+static uint32_t activity_last_uptime;
+
+#define MAX_IDLE_MS CONFIG_ZMK_IDLE_TIMEOUT
+
+#if IS_ENABLED(CONFIG_ZMK_SLEEP)
+#define MAX_SLEEP_MS CONFIG_ZMK_IDLE_SLEEP_TIMEOUT
+#endif
+
+int raise_event() { return ZMK_EVENT_RAISE(create_activity_state_changed(activity_state)); }
+
+int set_state(enum zmk_activity_state state) {
+    if (activity_state == state)
+        return 0;
+
+    activity_state = state;
+    return raise_event();
+}
+
+enum zmk_activity_state zmk_activity_get_state() { return activity_state; }
+
+int activity_event_listener(const struct zmk_event_header *eh) {
+    activity_last_uptime = k_uptime_get();
+
+    return set_state(ZMK_ACTIVITY_ACTIVE);
+}
+
+void activity_work_handler(struct k_work *work) {
+    int32_t current = k_uptime_get();
+    int32_t inactive_time = current - activity_last_uptime;
+#if IS_ENABLED(CONFIG_ZMK_SLEEP)
+    if (inactive_time > MAX_SLEEP_MS) {
+        set_state(ZMK_ACTIVITY_SLEEP);
+    } else
+#endif /* IS_ENABLED(CONFIG_ZMK_SLEEP) */
+        if (inactive_time > MAX_IDLE_MS) {
+        set_state(ZMK_ACTIVITY_IDLE);
+    }
+}
+
+K_WORK_DEFINE(activity_work, activity_work_handler);
+
+void activity_expiry_function() { k_work_submit(&activity_work); }
+
+K_TIMER_DEFINE(activity_timer, activity_expiry_function, NULL);
+
+int activity_init() {
+    activity_last_uptime = k_uptime_get();
+
+    k_timer_start(&activity_timer, K_SECONDS(1), K_SECONDS(1));
+    return 0;
+}
+
+ZMK_LISTENER(activity, activity_event_listener);
+ZMK_SUBSCRIPTION(activity, position_state_changed);
+ZMK_SUBSCRIPTION(activity, sensor_event);
+
+SYS_INIT(activity_init, APPLICATION, CONFIG_APPLICATION_INIT_PRIORITY);