Mms: prevent reply text being wiped from QMPopup when new msg arrives

If user starts typing a reply in the QuickMessage Popup, and another msg
arrives, the entered text was wiped. This commit saves the current reply
text, before adding/updating the new message, so that is doesn't get lost

Change-Id: I7aa4a488b233ed7554ec589fae0d615b18e3502c