optimize when msg.getCallback() not null

[wangyong0716]

leak could still occur when used like bellow:
WeakHandler handler = new WeakHandler();
Message m = Message.obtain(handler.getExecHandler(), new Runnable() {
@OverRide
public void run() {
//do something
}
});
handler.sendMessageDelayed(m, delaytimes);

although postDelay() should be used here, but what if when we get message like this:
Message message = Message.obtain(m);
and m has a callback?

so maybe sendMessageXX() methods should be optimized:

public final boolean sendMessage(Message msg) {
    if (msg.getCallback() != null) {
        return mExec.post(msg.getCallback());
    }
    return mExec.sendMessage(msg);
}

public final boolean sendMessageAtFrontOfQueue(Message msg) {
    if (msg.getCallback() != null) {
        return postAtFrontOfQueue(msg.getCallback());
    }
    return mExec.sendMessageAtFrontOfQueue(msg);
}

public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
    if (msg.getCallback() != null) {
        return postDelayed(msg.getCallback(), uptimeMillis);
    }
    return mExec.sendMessageAtTime(msg, uptimeMillis);
}

public final boolean sendMessageDelayed(Message msg, long delayMillis) {
    if (msg.getCallback() != null) {
        return postDelayed(msg.getCallback(), delayMillis);
    }
    return mExec.sendMessageDelayed(msg, delayMillis);
}