healthd: add register event interface

Facilitates registering events from external source files.

Add epoll events param to callbacks.

Change-Id: Icf8fb0c62af11453cbcc92877ff2c3bc03b9bb1b
diff --git a/healthd/healthd.h b/healthd/healthd.h
index dc3f67e..41dde13 100644
--- a/healthd/healthd.h
+++ b/healthd/healthd.h
@@ -68,6 +68,7 @@
 
 // Global helper functions
 
+int healthd_register_event(int fd, void (*handler)(uint32_t));
 void healthd_battery_update();
 android::status_t healthd_get_property(int id,
     struct android::BatteryProperty *val);