public void execute(Callback callback) {
        assert mAsyncTask.getStatus() == AsyncTask.Status.PENDING;
        mCallback = callback;
        mAsyncTask.execute();
    }
Executes the task. Should be called only once.
@param callback {@link Callback} to be called when done.
customtabs/src/android/support/customtabs/trusted/splashscreens/SplashImageTransferTask.java