For Android, the HeadsetState return null in Isolate mode

[Ted-Zhang]

the HeadsetState returns null during Isolate, The return value of _channel.invokeMethod<int?>('getCurrentState') is -1

Tested it on Android. and the isolate is spawn when receiving the background push notification.

Thanks

[shtreb]

@Ted-Zhang I created fork and fix this problem. May be it isn't apply for you, but anyone can see it.

@themobilecoder , Can you look at my fork?
https://github.com/shtreb/headset_connection_event