healthd: Implement scheduleUpdate

scheduleUpdate forces a healthd battery update,
which will notify all listeners.
Test: Manual (strace -p $healthd_pid)
Bug: 32645990

Change-Id: I8b19f79b633ca8988f0e3dd5be14757f6421ddcb
diff --git a/healthd/BatteryPropertiesRegistrar.h b/healthd/BatteryPropertiesRegistrar.h
index 095f3d3..14e9145 100644
--- a/healthd/BatteryPropertiesRegistrar.h
+++ b/healthd/BatteryPropertiesRegistrar.h
@@ -32,6 +32,7 @@
 public:
     void publish(const sp<BatteryPropertiesRegistrar>& service);
     void notifyListeners(const struct BatteryProperties& props);
+    void scheduleUpdate();
 
 private:
     Mutex mRegistrationLock;