conversation: adapt to libclient change in type of interactions

This is to follow the libclient (lrc) changes in the commit
9cb6b6115a39330521b98da0b67a7f9dece51bf4, where the type of
conversation's interaction field is changed from MessagesList
to std::unique_ptr<MessageListModel>.

Change-Id: If4e937bede3b958b8ae8523bff2489e8e6df593d