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 @smarek why handler is null need to be handled differently? for me, Message.obtain() call is the same as Message.obtain(null, what, obj), isn't it? so, we don't have to test for null, right?