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;
@zxw1962 
This is wrong as `handler` could be null. The alternative when that happens is meant to emulate a `Message.obtain()`, so the entire method has to stay as-is.