refactor(battery): Fire event on battery event.

app/include/zmk/events/battery-state-changed.h

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <zephyr.h>
+#include <zmk/event-manager.h>
+
+struct battery_state_changed {
+    struct zmk_event_header header;
+    // TODO: Other battery channels
+    u8_t state_of_charge;
+};
+
+ZMK_EVENT_DECLARE(battery_state_changed);