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/events/activity_state_changed.c

@@ -0,0 +1,10 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <kernel.h>
+#include <zmk/events/activity-state-changed.h>
+
+ZMK_EVENT_IMPL(activity_state_changed);