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 correct now, but what happens in the future if the underlying implementation changes and a check is introduced to `Message.obtain()`? 
Potentially, this will make all apps relying on AHC to fail if the device is upgraded to run a newer Android (with a different implementation for Message.obtain()).