added PendingRequestListener::onRequestAttached

Hi!

In our project we need an ability to attach to a pending request after device configuration changed. For example, a user wants to cancel a long running request or it's necessary to show a dialog to a user if there is a pending request still.
I saw the discussion about this in #335 but no activity on the feature. I'm not very experienced in android development, so please be patient about my PR :) If I did it wrong, please advice how to improve it.

Thanks,
Nick