BatteryService: fix FC on boot until battery stat is present 

updateLightsLocked() can be called from CM's added SettingsObserver
when battery stat is not present, causing an FC and a loop until
battery stat is present.

I/SystemServer(  502): Battery Service
E/System  (  502): ******************************************
E/System  (  502): ************ Failure starting core service
E/System  (  502): java.lang.NullPointerException
E/System  (  502): 	at com.android.server.BatteryService$Led.updateLightsLocked(BatteryService.java:735)
E/System  (  502): 	at com.android.server.BatteryService.updateLedPulse(BatteryService.java:709)
E/System  (  502): 	at com.android.server.BatteryService.access$1600(BatteryService.java:86)
E/System  (  502): 	at com.android.server.BatteryService$SettingsObserver.update(BatteryService.java:860)
E/System  (  502): 	at com.android.server.BatteryService$SettingsObserver.observe(BatteryService.java:822)
E/System  (  502): 	at com.android.server.BatteryService.<init>(BatteryService.java:187)
E/System  (  502): 	at com.android.server.ServerThread.initAndLoop(SystemServer.java:328)
E/System  (  502): 	at com.android.server.SystemServer.main(SystemServer.java:1309)
E/System  (  502): 	at java.lang.reflect.Method.invokeNative(Native Method)
E/System  (  502): 	at java.lang.reflect.Method.invoke(Method.java:515)
E/System  (  502): 	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:781)
E/System  (  502): 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
E/System  (  502): 	at dalvik.system.NativeStart.main(Native Method)