refactor(power): Extract activity/idle detection.

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

app/include/zmk/activity.h

@@ -0,0 +1,11 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+enum zmk_activity_state { ZMK_ACTIVITY_ACTIVE, ZMK_ACTIVITY_IDLE, ZMK_ACTIVITY_SLEEP };
+
+enum zmk_activity_state zmk_activity_get_state();