Fix comments typo, rename param, refine obtainMessage

1. fix typo of omit;
2. change param name from "value" to a meaningful "sync";
3. seems obtainMessage can be refined as one line;
@fineswap future changes to this API is unlikely, as this is stable behavior since API v1
@zxw1962 I'm sorry, I've gone too quickly through implementation of Message API, this is correct change, as the [`Message.obtain(Handler,int,Object)`](https://developer.android.com/reference/android/os/Message.html#obtain(android.os.Handler, int, java.lang.Object)) just sets passed values into internal fields of Message. I'm pulling back my previous complaints.