Fixed crash on some Android devices.

On both my Android devices running 4.1.2 Jelly Bean i faced the crash in OnSignIn() event. This was due to following error in logcat right after OnSignInSuccessful event:

```
E/hxcpp   (1578): GetLocalAllocMT - requesting memory from unregistered thread!
```

Im using OpenFL 1.2.2 and hxcpp 3.0.2.

I've done some googling and found thead in haxe mailing list with the exact same problem: https://groups.google.com/forum/#!msg/haxelang/_l2LP0lY8JM/3rHfn5SEueMJ

So, just as a workaround i used the suggested code in OnEvent method  and recompiled .so library. Now it is working like a charm without any crashes. Please take a look and i'll be very happy if this fix will be merged.

Thanks.